You do tend to get several turns before another family members turns up. But it's hard to assassinate members simply because any failure and they quickly gain traits to protect against assassination attempts. The plus side is you can drive them insane by repeated assassination attempts. In my current Milan game, Venice has 3 family members. Two of them are insane.

Also noticed that getting caught trying to murder someone *massively* drains your reputation. Far more so than declaring war due to an attack. So with this in mind it's hard to effectively use assassins against a neutral or allied faction.