Confused Help..My husband began sleeping with his sister's husband sister

Ok here I go! My husband and I have been together for 8 years and have 2 children together.

I can't seem to get over the past so here's my story.

At the beginning of our relationship 6 months in my husband began sleeping with his sister's husband sister.

This relationship continued for about 1 year and a half. I have to see this woman at every b-day party, family gathering, and everytime I do I get pissed all over again and I feel stupid for still being with him.

I love him and I don't believe he has cheated on me since with her or anybody else.

I'm just wondering why I can't shake these feelings and how should I handle my interactions with this woman?