Is it weird to go to my boyfriend's ex-girlfriend's birthday party?

Let's call her Janet. We met in our dorm freshman year and talked every once in awhile. We were "hi and bye" friends for freshman and sophomore year. Junior year, we rarely saw each other or talked. Senior year, she was dating my current boyfriend. Since me and him were friends at the time, all of a sudden, she was extra friendly to me and would talk to me. They broke up and he's with me now. However, she still talks to and is friendly with me and my sister.

Anyway, she sent me a text, inviting me to her birthday party, which I thought was weird. I'm thinking of going because I'm interested in seeing the club she's having it at and I'm not doing anything during that time. So it would give me something to do and I could meet some people and just have a good time.

My sister thinks I shouldn't go though and thinks it's weird. Is it? What would you do? Any advice would be appreciated :-)