You have a few choices.
1. Tell SIL that you will no longer be doing the joint party because you do not feel that you Xh bringing his pregnanat mistress is proper for the occation. And then have your own small gathering. Your child will ask questions, your X will probably have words for you, but whatever.
2. Continue with the jointness, letting X know that while you understand the OW is here to stay, that you would prefer not to have her at this communion as its still very soon for you....if he's and ass about it, then see number 1.
3. Go to the party, and look GOOOOOOOD....find a male friend to go with you, or find some female friends that can provide you with support when you need it. But dont let that OW see you flinch.
Honestly - the best reveng is living your life like they dont exist. Giving them the power to make you change your plans is to much power.