What should I do? Let them be responsible?

I have an older brother who is currently seperated from his girlfriend, the problem is that they have a 7 month old son. She has a 4 year old daughter. They don't take care of these kids at all. Now that they're separated she doesn't want him to see the kids, which I think is childish and not necessary. Well because she goes to school and works part time my husband and I watch the children. We both work full time and go to pick up the kids right after work, then she picks them up or we drop them off usually about 10pm. It's been every day for a while. So this is my thing these kids are better off with my husband and I, the reason I say this is because everytime I go pick up the kids they are in the same clothes as I left them in the day before, I have to take them baths. Besides that I don't have a lot of their things at my house so I have to go out and buy them clothes, diapers, socks, etc. almost everytime they come over. Because of that I would rather have them as much as I can that way I know they have what they need and are taken care of. My brother is almost 30 and completely irresponsible, she's 25 and same thing, not responsible unless it comes to work and school. Those are the only 2 things she cares about, it seems like the children she could care less about though. It's been like this since the day the baby was born for the past 7 months either my mom, my sister, or me take care of them. If we don't then they don't bathe for days, and who knows if they even eat. When we pick them up they are starving, both of them want to eat everything. My question is should I keep doing this? Do I keep taking care of them? I'm afraid of what will happen if I don't, but do you think they'll ever be responsible for their own children? I welcome all of your opinions just please let them be thought opinions and not just criticisms. Thank you.