I have my doubts Uriel's problem was that their mother died in childbirth. It could be, sure, but the way I read this there seems to be more to his situation than just plain neglect. I think it'd be quite interesting if it turns out that Uriel is also a child born out of wedlock rather than legitimate.
It would also create a counterpoint to the other child they mean to kill, and a moral conflict in Lamia herself (and upend her world) that it would be pretty nice to explore, too.