Your first question is about half a GM problem, and half a player/PC problem. I've been on both sides of this issue: GMing for a character who had little reason to participate in the story with the other PCs, and playing a character who had no reason to participate in the story and every reason to run wildly in the other direction. In both situations, the solution has two parts:
The GM finds ways to include the PC in the plot.
It sounds like your group is already doing this, but for posterity: this means that the GM must look for ways to actively involve the PC in the story, by using their background hooks, talking to the player about what the character might find interesting, and otherwise looking for ways to help the character find motivation within the story to participate in the story.
You do need to be careful not to change the story or the game's focus so much that the other players begin to feel excluded or ignored, but this is a matter of understanding your group, finding a balance, and getting buy-in from the player of the problem PC. Which brings me to part 2 of the solution:
The player finds ways to include their character in the plot.
This is the much harder part of the solution: the player must meet the GM halfway, otherwise the game becomes "The Problem PC Show!" and no one else has fun. This may mean the player has to bend or tweak the character in some way - not enough to compromise the core of the character, but enough to keep them with the group when otherwise they wouldn't stay.
This is, admittedly, difficult to do. Some people are so attached to their characters that they're unwilling to compromise; or they simply can't see a way to bend the character on an issue without breaking them completely. However, it's absolutely required in order to solve this problem.
Why player buy-in makes a difference:
When I played a character who had no reason to participate in the story, I looked hard for ways to make her want to be there. But all logic dictated that she run far away, find a hole, and pull it in after herself, so any other choices felt wrong to me. This showed in my roleplaying, and ultimately caused much frustration for our whole group as session after session became centered around getting her involved when she didn't "want" to be.
I put "want" in quotes there for a reason. Players with especially strong "my guy" syndrome (which I know I'm prone to) will insist that they're "just playing their character" and that "the character can't be changed". But when I GMed for a character who had no reason to be there, his player was willing to meet me halfway, and the result was a fun campaign for everyone where that character even made a large part of the highlight reel.
The reason it worked was because the player was willing to say, "he has no reason to come along, but does anyway, because that's how group-based RPGs work." We hand-waved it a bit as "he has nowhere else to go and nothing better to do", but really that character should have been as long-gone as mine wanted to be. But because the player was willing to take the meta option and bend his character enough to say "screw it, he's participating" without looking for a story reason, it worked. It kept the game from focusing too much on that character, but meant he was still there when story developments happened that he was interested in, thus giving him the time he needed to organically grow interested in the plot.
When the player isn't interested in the scenario
This part is a little trickier. If the player doesn't buy Dresden's world, including all the magic elements, what's he doing playing a Dresden Files game anyway? You and your co-GM need to talk to this guy privately, out of game, and ask him why he's playing. If he just wants to hang out with your group and doesn't care much about the game, then you need to address that. Maybe give him a character with a minor support role, so that he doesn't need to participate much in the game and can just hang out; maybe say that if he just wants to hang out, the game isn't the time for it and you'll find other group activities he can participate in. It's up to you and your group to decide what's best here.
If he insists that he wants to play the game, then you need to find out why he's actively sabotaging it. Tell him that his actions suggest he doesn't want to play, and in fact are making it harder to play. It's possible you simply have an attention hog on your hands, in which case you should deal with him appropriately. (I'd suggest going through the problem-players tag, as there are a number of questions and answers you may find relevant depending on your exact situation.)
TL;DR: The GM and the player need to meet halfway on adapting the character and the world. But if the player isn't buying the game's scenario in the first place, then that must be addressed first.
Yes. You should leave.
These people are not respectful, and are not worth your time. Tell your brother you're not interested, and stop joining the call — he's the only person you have actual contact with, and given the behaviour of the rest of the group you're best cutting ties with them completely. (Who would want friends like that?)
You could expend effort working with them to overcome their talking over you, their negligence of you getting value from the game, and their aggressive and disrespectful behaviour toward you and making you feel uncomfortable — but that's a lot of effort, not likely to succeed with this group, and not really worth it when there's a world of gaming groups out there full of people who are quite ready to treat you and others respectfully like anyone ought to. (The fact they're making rape jokes in front of a rape victim is appalling, and one of several strong signs they're probably not interested in engaging with you constructively on resolving any of this. Indeed, a lot of what you've described reads to me as abuse.)
We have some guidance available on finding other groups that you might find useful, kindly gathered by a resident moderator:
Bad match with a gaming group, how to leave? provides general advice for politely leaving groups, but in this circumstance, you don't need to use it. The group does not need, and won't be receptive to, an exit conversation. (Your brother may be different in this regard, but you'll need to judge that for yourself.) Other people in the future may be receptive.
You mentioned these people were in the furry fandom — I'll advise based on personal experience that this behaviour is completely non-representative of that fandom, and exceptionally awful and heinous bullying by any standard, including theirs. Individuals in the furry subculture may be somewhat more inclined to poorer social mores than many groups, but the subculture is largely made up of harmless, well-intentioned, friendly individuals.
Roleplaying games should always be fun. If you're not having fun, talk and work with your group to resolve the source of un-fun, or leave and find another one. You found this one, you can find another.
(NB: In a previous version of this answer I'd unwittingly given some advice that appeared to suggest this behaviour was normal for people in the furry fandom. It isn't. At all. It's awful, and entirely outside any behavioural standard of the subculture.)
You must, in an out-of-character context, talk about this with your GM. There is no rule in the world that will solve player favoritism. This conversation needs to include, bluntly
Feelings will be hurt, if it has already gone this far.
It is already too late if it has come to asking strangers on the internet.
If you catch this earlier, the answer is much different. If you catch this early, you still have the out of character conversation, and you still do not sugar coat anything, but there is no ultimatum. You come up with a mutually agreeable solution. sometimes that is strict adherence to the rules (which may require a change in system), and sometimes that means asking her not to bring her husband. Feelings need not be bruised then. In your case, sadly, there will be hurt feelings. The cardinal rule of the hobby "these are your friends, don't be a jerk", has already been broken.