Recently tried something, and it worked.

In extended empires, one tends to find a scarcity of family members in certain areas, and a lack of them to bribe as well.

One way to help alleviate this is, when that message pops up about a possible adoption, or marriage into your faction, before clicking yes, relocate the capital - then click yes, before moving the capital back. One could also do this with normal heirs, but that would require making a note of who's coming of age in the next turn, plus it means a turn with the capital in a non-safe position - leading to possible revolts etc.