Pretty much all rules dictating deity worship are gone in 4e, replaced with DM discretion and recommended threats of story-based consequences for unpopular actions or beliefs within a subculture.
Additionally, there is nothing in the rules preventing a character from worshipping multiple deities, cherry picking which parts of each that appeal to them. The PHB openly states, “Most people revere more than one deity” (PHB pg 20), but most people are under the impression that, because the online tools only allow you to select a single Deity, that players can only worship one. This is point-blank untrue, and furthermore, wouldn't make much sense within a provably polytheistic society.
So your character could worship Sune because she embodies the ideals of Beauty and Trickery, and another evil deity of your choosing that embodies torment and madness. The two would combine nicely in your concept.
Another twist I've used before successfully is a character who believes that she worships one deity, but who is in fact interacting with another entity entirely.
Perhaps the most well-known iteration of this kind of story arc is Pelor the Burning Hate, but it doesn't have to be a universe-wide deception by the deity, it could just as easily be a case of an individual or single temple being fooled by a fraudulent impersonation of their intended deity.
In this case, such a character could honestly believe that they are worshipping Sune, but their prayers are being intercepted and answered by Cyric, the Prince of Lies, who is attempting to gain enough influence to break free of the prison that Sune helped build for him.
His alignment could have aided in the slow corruption of your PC, and the illusions afforded him by the trickery domain would be neatly augmented by the madness and strife slowly creeping into your PC's demeanor.
It's up to your DM how to adjudicate things such as feats, paragon paths, or epic destinies which require one to worship Sune, but thankfully there's only one of each, and none of them are stellar picks.
Just because you want your character to be a follower of Bahamut, that doesn't necessarily require all his people to follow Bahamut.
For example, he could have been rescued by a follower of Bahamut, he could have had some kind of vision, he could have touched a holy relict of Bahamut that created a special connection between them, etc etc etc.
Also, your character is a Draconic Bloodline Sorcerer, that's a ton of story hooks available right there, depending on the background story of why that's your bloodline.
Although, as a side note: depending on how much you want to go Bahamut, the Divine Soul Sorcerer from Xanathar's Guide To Everything, page 50, might be relevant for you as well.
Anyhow, the bottom line is: think outside the box.
Just because your character isn't a dragon (for obvious reasons) or a Dragonborn doesn't mean he can't be a follower of Bahamut. There might be some lore out there about non-dragon(born) followers of Bahamut*, but the easiest solution is to just create it yourself!
*I'm pretty sure that'd be previous editions, though. I don't think something like that is mentioned in the 5e rulebooks.
Best Answer
There are no rules for worshiping gods, but likely a lawful good god
Aasimar are found in the Volo's Guide to Monsters and the relevant part of their description is below:
The description is "champions of the gods", gods in general with no specific reference made to a particular deity. The only guidance we are given is that Mount Celestia is home to mostly lawful good deities. The next section also assumes this:
Although this is descriptive text the assumption is that most Aasimar will follow a lawful good deity. This doesn't have to be the case however, since the description goes on to mention their conflicted souls:
Remember however, that this is only guidance and there is no restriction on the alignment, deity or class of any race in 5th edition. What gods exist in your world is up to your DM and you can work with them to decide which suits your character best. If you choose to be an archetypical aasimar that would likely be a lawful good god but you aren't required to follow this.
As for celestials, they are a creature type not a race. Therefore guidance on what god they are likely to follow is hard to find. The stat entries for most celestials are lawful good, though some (Pegasus) are chaotic good.
Generally celestials are good aligned and would worship a good or lawful good god if they worship one at all. Despite being basically angels themselves celestials are no more or less required to worship than any other creature type.