I've stopped accepting adoptions or men of the hour on principal unless my current king has one or two sons who are of age, as otherwise some randomer who you just adopted becomes next in line for the throne! I got caught out by it badly on my current English campaign when I wasn't aware of the mechanics used with adopted son, so now I have a string of two or three adoptees in line for the throne as my King's first-born was as-then not of age