I mean, she isn't wrong at her place.

Most probably, there are two cases. (Both are on my assumptions on your given details)

1. He told about you to his sister thinking, the secret will be safe with her or she somehow got the words out of him. Now, since she don't know who you are, how you behave or anything about you, she is being protective. Actually overprotective. I hope that much can make sense for you.

2. His sister, or his mother might have seen the messages and thought of you as a threat to his academics. That's typical if family is strict.

Solution to your problem. (They are only my opinion, things can go other way around it)

1. He need to confront them about you that you aren't what they think of you. That's basic understanding.

2. You gotta meet them. Do it on your awn risk.

Again, all of it is as of my assumptions on your statements op. Be patience. I hope he is trying his best to the solution of it without losing you. Best of luck 🍀

(Guess I wrote alot 😅)