Bytheway I sympathise with how you feel. Small children are very wearying, especially when you finish your working week and are looking forward to a relaxing weekend! I admire your husband's kindness and stamina in looking after them so frequently, but I think you need to negotiate fewer weekend visits - perhaps just once a month. Your husband could still have them as often as he wishes while you are at work. Your S & DIL are a lot younger than you and should be able to cope with 3 small children at the weekends.
When my own children were small I often had half the neighbour's children in our house after nursery/school and at weekends. But I was a lot younger then, and the noise, mess, arguments & constant activity didn't bother me. But now I find it exhausting. I love my GC and AC dearly but I do not want to babysit, childmind or care for them for any length of time. Been there, done that, it's their turn now. I'm quite happy to enjoy their visits/short stays and then breathe a sigh of relief as life returns to normal when they leave. They don't love us any less than their other GPs whom they see more frequently.