It's the OP's sister, not SIL.
OP, I must admit, in reading your post I'm confused at what the predicament is that you feel you're in with regards to your MIL and your shower
1) You've never had a relationship with your MIL.
2) Your DH doesn't have a relationship with his Mother.
3) She is disrespectful towards you and DH.
4) You haven't spoken to her in 2 years
5) You don't want to be tense or uncomfortable with her at your shower.
6) She may cause problems if she comes to the shower.
7) She hasn't called to congratulate you on your pregnancy.
It's very clear that you shouldn't even think twice about inviting her to the shower, or feeling guilty if you don't.
Don't sweat it!