I'm seven months pregnant with my third so can totally relate to the irrational worrying about this- just blame it on the hormones!
In all honesty you shouldn't worry hon. Your sil has had four babies - she knows what it's like! Ask your mil if she could have your other two for the first few night you are home then they can keep all the kids over there - there are two of them so they should be able to cope with six between them and remember what you don't see you can't worry about! If it all gets too much with visitors just take yourself off to bed with the baby they're hardly all going to pile into the bedroom with you. That's what I'm planning to do!
(SIL is staying at MIL's house right? If SIL, her four kids and MIL are coming to stay at yours, move house and don't give them your new address!!!)
SIL should be well used to looking after her four children so she won't necessarily want to dump them on MIL and run, and she can even help with your two for a few days after your birth.
As for visiting you can always ask that adults only visit at the hospital and that children have a brief visit to meet the new arrival and then all go off together to play with one of the adults so that you have a break.