My girlfriend asked me if I wanted a threesome with her twin sister. What do I do?

A couple hours ago my girlfriend asked me if I would want to have a threesome with her and her twin sister.

At first I was all for it, cause like... why wouldn't I be open to that.

But then I started to think about it. And I kept wondering if this would even be a good idea? Would it hurt our relationship?

This is going to make me sound terrible (which is why I asked this anonymously) but I really, really want to do go through with this.

So I guess my question is, how do I go through with this without any problems later on?

TL;DR girlfriend asked if I want a threesome with her and her sister. How do I this without hurting our relationship?
My girlfriend asked me if I wanted a threesome with her twin sister. What do I do?
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