How to tell my sister I can't go to her party today?

I always feel bad I'm a very empathetic person so I feel extremely bad doing things to people no matter how mean they are to me . Okay my sister is having a party today but my boyfriend is my only ride and I don't really want to go in the first place since I'm very introverted however , my boyfriend is bipolar and has fallen into a depression and he doesn't feel like being around anyone not even me. So I just don't know what to say to her? Because it will seem like I'm lying.