It really does sound like she has not learned any play skills. She is still doing toddler things like knocking towers down just because it is fun. How many of the big kids will still be around in September or will some be full time school leaving some that will need a playmate. The one year old coming back will also be older and have more skills so that will help too. How does she seem as a mother's helper. If she has no one to play with you might be able to make good use of her when you have your baby for getting you things and sitting nearby to watch etc so she feels useful even though she isn't really doing much. Does she take instruction at all in terms of showing her proper social skills and then practicing them on the big kids. There is also a point where you need to just leave her to play on her own and it isn't until she gets really bored of that that she will want to seek out better ways to join the others. Make sure when you do get a chance to interact that you do something with the whole group like a game where she can participate as a social equal so both sides can see what she can do.

But is it worth keeping her and the frustrations when you have your own baby to deal with only you can tell that. It might be a good idea for her to be with some peers so that if the issues don't resolve themselves quickly they will become obvious and teachers will want to deal with it. I have sent kids in the past to a daycare centre for similar issues because the parents were in denial so once the child was with their peers it became obvious they had severe delays that needed attention beyond the wait and see. Can you fill the space though if she goes?