I have a pretty good headcanon for this one:
As it seems, the daughter is trying to get obsessively close to MC, and why is the mother seeming to help her? It's obvious that it is all part of a plan that they have been cooking for a long time.
My wild guess is that in Ch. 21.5 , when the mother reminisces about the past, she looks REALLY young, the braided twintails could be implying that she is still a student. The guy she's with could be her husband, but I think that that's HER (the mother's)STEPFATHER.
Yup, it seems really far-fetched, but if you think about it, then the daughter could be a child of the mother and the mother's stepfather, who were in a relationship. and because she became pregnant while too young ( it is told in a chapter which I don't remember), she is constantly hospitalised and needs healthcare (also implied at the start of the manga).
Now, for the daughter, she IS trying to seduce the MC. The mother and daughter are very well aware that the sick mother will not be able to healthily go through a pregnancy. Also, it is very likely that the daughter knew about the true relationship of her biological parents. And now she (the daughter) is trying to do the same what her mother did, or something like that. And for that to happen, they (the mother and the daughter) are working together, cutting the ties of the MC with his family, so that they don't interfere.
So, my guess is that this manga is gonna become even more f*cked up as it progresses.