Fulfilling vassals’ ambitions gives no opinion bonus. Is this a bug

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.

Yes, all children born of a matrilineal marriage will be of the mother's dynasty. Therefore a couple things come to mind that might cause what you are seeing.

1. you made a matrilineal betrothal but the AI offered a traditional marriage when the characters came of age and you accepted. AI likes to do this, its very sneaky. You can double check if the marriage is matrilineal based on the overlapping rings on the character screen. Even if the AI does try this you can just reject the offer and send a matrilineal offer and the AI will accept.

2. The lord had a bastard child and legitimized it, or had children via a previous marriage. Check if the mother is in fact the member of your dynasty.