It's a sad state of affairs, the whole in laws, step kids and ex's shananagans. I've been a step mum for 8 years now, dsd is 12 and dss is 9, we are very lucky in that we get on really well with the ex but it's still been hard b'cos IL's prefer the ex over me, I personally have come to terms with them not bothering with me, I have my own family and the are very hands on when it comes to our ds who is 4, I can honestly say that my IL's prefer my step kids to my son, who let me remind you all have the same father, we went through a lot of arguements when mil always looked after dsk's but never offered to have our son, she rings the ex regularly to ask how she is but hasn't rang us in years.
I have always stepped aside and let dh deal with it but it all came to a head about 18 months ago and dh stopped going round to his parents and they haven't seen our son in just as long, they don't even know I'm pregnant with our second child and they will never be invited to our house ever again unless they decide to make a huge effort in seeing our son.
The IL's still see the stepkids regularly and talk to ex regularly, which I don't resent b'cos stepkids and children too and just b'cos they don't see our son, we would never stop them seeing the other 2. Plus its completely up to the mother, when it comes to situations like this the father never has a say in things, but then if he did it would only be the kids hurting and we would never want that.
Anyway to get back on track, I have a lot of sympathy for you, it's very hard to have to give your life for your stepkids but then not have a say in anything. I don't want to turn everything into a doom and gloom but don't be surprised if they don't even bother with the child you're pregnant with now, when our son was born 4 years ago we always had to do the running around for them to see him but when we put a stop to it and said 'you make the effort' thats when all contact stopped and they haven't seen him since.
If your IL's want to see the child you are pregnant with now then they have to understand that your dd's come with the package and they should never treat them differently, the same as you do not treat your dsd differently.
As for the sil, it really isn't worth it hun, she sounds like she just wants to get the boot in, it's taken me 6 years to deal with this whole situation but I'm at a stage now where it really doesn't bother me anymore and I can let it all go over my head.
I really hope you and your dh get some closure on this whole situation, as no matter how stressful it is for you, times that by a hundred fold for your hubby.
Big hugs xx