What should I do?

My boyfriends niece and nephew are 4 and 1 years old, his niece (4year old) caught nits from her nursery quite a while ago now and her mother doesn’t even try to get rid of them? She has thin blonde her so they are extremely noticeable as well and she’s passed them over the the 1 year old boy who’s scratching his head so much he’s cutting his head and bleeding! I have just got back from seeing them and just seen in the mirror that I have them now I obviously got to close without realising but I feel like such a tramp this isn’t the first time I have caught them off them either but their mother does nothing about it for them! I also now have a baby on the way and I have said to my partner if she won’t start getting rid of those nits my baby won’t be going near them, do you think I’m being too harsh? It honestly makes my skin crawl how she can just leave them like that even though plenty of people have told her she needs to get rid of them!