[Progress! This was good. He doesn't think they've ever been on the same page before.]

Okay, good. So based on what you just told me, this girl is dating your brother and asks if that makes you sisters.

I can understand where that is a sensitive topic for you, but if this girl does end up marrying your brother, what is she? She's your sister-in-law, which is a type of sister. Is she the same kind of sister that you have back home who I'm sure you're missing and trust with everything? No. Does it matter that she's not technically married to your brother yet? Not really. In-laws are a very tricky part of family politics, but they're valid.

So unfortunately, kiddo, you were in the wrong here. It sucks and I know she accidentally hit on a bad topic, but the punishment didn't fit the crime. Shooting her down like that was wrong. And have you noticed, I haven't said a word about her knowing the intricate details of why you snapped at her? It's because it doesn't matter. This isn't about her and your sisters. It's about you and her. She's your sister-in-law and that was her attempt to welcome you to the family, not attack you.

However, there are three other parts to this: the issue of you not trusting her and why, whether or not you can give her a genuine apology, and the fact you've removed yourself from the equation which has me concerned.

Want to talk more about the sister thing or want me to keep going? What would be most beneficial to you?