Ellie I sympathise. I have a friend who I love, get on really well with, who comes for 3-4 days over Christmas and I can't wait for her to go! I know it's just me but I find entertaining visitors stressful. I know that I should relax, like others seem to do, but I just can't as I feel personally responsible for them enjoying themselves.
For you, depending on how long they stay, definitely use some of the suggestions above plus build in some sanity-saving stuff for yourself, as I am learning to do. If the men don't cook, have a least a couple of nights with takeaways. Pay for them but get the girls to choose and make the list and get your SS to collect them. As for SS being tired, presumably he's not an OAP too! Okay he's a single parent but surely a day's rest should set him back on track. So arrange an early day out with DH and GC without him so he can recharge his batteries. Take the GC to a recommended indoor play area so they can bounce and slide away their energy. It's weather-proof, many suggestions on Groupon, and you can buy hot lunch and don't have to cook. Sandwiches when you get back or pick up a pizza. Don't forget to say to SS 'we're taking the girls to give you a chance to catch up on washing etc. Get your own meals, plenty in the fridge'. Arrange long days out so that the girls are tired when they get back, SS can enjoy parenting, and they all have some lovely memories. Sometime towards the end of their stay, arrange a full day out just for you 'that you really can't avoid, so sorry'. Don't explain or say where you are going but make sure you go, even alone. Recharge your batteries and make sure you don't do anything at all that day for them. Tell OH, SS and GC all together as soon as they arrive which day you are going out so that they have time to think of a plan for that day. In the days in between don't be afraid to say 'I'll clear the dishes, you wash them/stack the dishwasher', '** lay the table', 'clear away your toys and then we'll go out'. It takes a bit of training yourself, but insist, children actually like a bit of order themselves. You will have to be proactive, assertive and insistent as it seems that no-one else is going to be.