Yup, noticed it in RTW too alpaca.

In both Rome and M2TW though, you need to assassinate all the newest family members first though to eliminate the faction. Reasoning is that the newest take up 1 slot in the limit of 4. AI also only gets 1 general a turn (except through MOTH but they hardly win battles against the human) and if you keep killing that one, eventually all the family members above them in the tree will have had their max number of children (save for those who are still underage) and then you can go kill any of them at random until one kid comes of age or they die out.