What does adopting other nobles do for my family?

[Falesz]

Whats the difference between a statesman being a family member or just a party member? What good does adopting them into my family do? (Besides the +10 gravitas boost, which is quite unimportant)

[Alwyn]

The cumulative gravitas boost of a lot of family members can give your faction useful bonuses for high influence.

One way to a get a lot of family members is to adopt members of your party into your family, then use the intrigue which gets them a spouse (and to promote them and, when their kids come of age, promote them and find them spouses too).

When the kids grow up, you can have a pool of free generals who will usually be loyal because they're in your ruling family (however, occasionally a member of your ruling family might found their own rival party).