sister in law won’t take no for an answer

My husband’s mother just passed away. I’m trying to be a very supportive partner during this time on top of dealing with my own stresses. I have countless appointments, homeschool my oldest, I’m a full time mom to two young kids and I’m pregnant. I’m dealing with health issues as well. Well my husbands sister wants us to take in her sisters 3 young children for the rest of the week. I can’t handle juggling 3 more kids right now. I have appointments I need to get to and that’s 3 extra people I need to cook for and clean up after. I told my husband yesterday that I’m sorry but I can’t. His sister doesn’t understand that my husband isn’t going to be the one taking care of them, I am. I don’t have the energy nor the time. One is a literal baby. Today his sister continued to push telling him he has to do it. I told him while he’s working who’s taking care of them because I have appointments I need to get to, unless he’s willing to take more work off and take care of them because I can’t. I understand their family is going through something very difficult but I have my own things going on that don’t halt unfortunately. My husband works everyday all day and barely helps me with our kids let alone 3 more. I can’t.