Someone used language to describe my polyamorous relationship that I find dehumanizing

My boyfriend is new to polyamory and was explaining things to his sister and she asked if he and my girlfriend were "sharing me." That feel like really gross and dehumanizing phrasing to me; am I overreacting by being unhappy with that phrasing?

It's okay to have feelings about things; and the way other people describe you is something that most people have strong feelings about! You can be unhappy with whatever you want - the word is overreaction, not overfeeling, for a reason. It would be an overreaction if you forbade your boyfriend from ever speaking to his sister again or ordering 100 t-shirts that say NEVER USE THESE WORDS TO DESCRIBE MY RELATIONSHIP, PLEASE and only wearing those.

In this case, it's a third party to your relationship - your boyfriend's sister - who used a phrasing you find icky, in a conversation with someone else. Probably best to leave that alone for now - if someone else uses that language when asking you about your relationship, it's totally fine to say "actually, we don't like to describe it like that," and then give your explanation of what polyamory is and is not, for you.

It's totally okay to bring up with your boyfriend that you don't like language that frames you as an object or possession to be shared - not as a thing you're angry about or an issue you have with his sister, just as a heads-up that you're not comfortable with it. Just like I might say "hey, thanks for going grocery shopping, but in the future, I like creamy peanut butter, not chunky."