Having three kids and working at a Daycare and being 15 weeks pregnant is stressful and then on the weekend one of my family members wants me to keep one of their kids I'm very much drained and can't get any peace and quiet time 2 myself...
Just let them know that at this time your plate is too full, you have to look after yourself too so you can take care of your babies. If they get upset and choose not to try to see it from your point of view then they didn't deserve you giving up your time to look after their child in the first place.