Crusader Kings 2 – How to Check Opinion Between Any Two Arbitrary People

You have a couple of options, none of which are ideal or guarantee success.

• The most straightforward is to simply continue murdering the heirs until an heir of your choosing appears as the top choice in the election screen.

• Another way is to gain an electoral title in Scotland. Look at their de jure counties and try to snatch one in a war. Once you've done that you can nominate a candidate for the Scottish throne as well as see their second and third options for the heir, which can help with the murdering option I mentioned above

• Thirdly, if you are up for a major war, see if the brother you talk about has a heritable claim to the throne (weak or strong). Strong heritable claims are great as the children will be of your dynasty (matrilineal marriage) and have the claim. If you can't get them elected you can go ahead and press their claim. T

A nice addition to this is if you make your own kingdom also elective and then nominate the new king of Scotland (now of your dynasty if the above succeeded) as your heir. You can then join the two kingdoms together.

After some research, and vigorous testing at the labs, the results are finally in.
TL;DR: Affine was right, you can live in peace and harmony with your ambitious vassals. Presenting some evidence.

• Duke 1 is ambitious, but doesn't want anything from me. There's no penalty. (profile card)
  
• Earl 1 is ambitious and title claimant, but doesn't want anything from me. -50 penalty. (profile card)
  
• Earl 2 is content, but desires one of my duchies. -20 penalty. (profile card)
  
• Duke 2 is content, but wants control of one of my counties. -25 penalty. (profile card)

All of these guys are my direct vassals, and none of them have any inheritable claims to the titles they desire/want control over.

Experiments:

• The reason why Duke 1 doesn't want anything from me, is because I have nothing to give him. He controls the other counties in the duchy directly or through vassals. If I retract a vassal, he will want control of the county (-25) AND the ambitious penalty will appear (-50). He is not a happy camper.
• I created a kingdom title, which de jure holds his duchy. He now wants to become king, and all penalties become active.
• The situation for Earl 1 is a bit complicated. The de jure duchy is outside my realm. He does actually have a -25 opinion penalty to the holder of the duchy for "desires county". Since I can't give him that county, it's a bit unfair of him to hate me so, I feel … unless ...
• This woman is unlanded, but also hates my guts. Like Earl 1 she is ambitious and also a title claimant.

Conclusion:

• Any vassal, content, ambitious or not, may want control of a title. This will trigger a negative opinion modifier of 20-25.
• An ambitious vassal will not automatically have the huge -50 opinion penalty.It will however trigger once you have something he wants, and will then stack with the penalty of "desires/wants to control X".
• Ambitious title claimants will also have the -50 negative opinion modifier.
• Any ambitious character will always have a -5 opinion of any other ambitious character.

This means, that you can have a reasonably good relationship with an ambitious vassal, as long as you keep him happy, and he doesn't have a claim to your titles. Depending on how you structure your realm, this might be feasible. However, once you have something he wants, you might rack up to -80 in negative opinion immediately (-50-25-5). This will make him rebel very fast.

Furthermore, I am almost certain that characters with the ambitious trait are more likely to plot and join factions against their liege.

Tips:

• Avoid ambitious vassals, unless you are prepared to appease them or at least make them vassals of one of your friends.
• Never, ever give landed titles to ambitious people who already have claims to your titles. Not only will they hate your guts, but you will also see the "Count Jimmy for Ireland" factions pop up. In fact, I would perhaps consider invoking tyranny just to relieve them of their lands.