It doesn't always work like that.
I have nephews (in law) who went into care due to neglect, parental domestic abuse and substance misuse. Dad had been in and out of prison for serious offenses. Mum had always made questionable choices in life but on her own was a fine mum but unfortunately, always chose men over anything else and this partner was not a good influence.
Anyway, the boys were put into temporary care initially. Other family members were approached by social services but we weren't. Presumably because the parents didn't identify us as an option. We are a stable family with sufficient funds/support network etc. so there's no logical reason why they wouldn't ask us, but they didn't.
We didn't reach out, not because we don't love the boys (although see them very rarely), but because we have our own young children and having our nephews would bring chaos to our door as there's no way their parents would respect boundaries. We avoid them at all costs as they're very bad news.
The boys went into care and are soon to be adopted. It's really sad and I do feel bad as they are lovely boys who deserved so much more, but there's no real way they could have come to us.
So the fact that Constance's family didn't take them in doesn't necessarily mean they had the option to. It could be for lots of reasons. At that stage, constance was estranged from her family, it's possible social services didn't even approach them.