I'm going to visit my sick dog at my stepdad's house after telling him I had an issue with his fiancee
Why are you having issues with your stepfather's fiancee?
She always has other people over, I feel uncomfortable.
It's not uncommon to not get along with a parent's new partner. If you're at their house though then you should be respectful about their rules in order to not create drama within the family. If you do create drama, your stepdad will be the one that has to deal with it.
I'm going to just not say anything then.
I think that might be the best course of action. You could also talk with your dad about how you feel. He might be a bit more understanding and maybe have set aside time for you to be there alone.