It sounds like you are overreacting. Obviously she speaks to you, if you notice she is happy when speaking to you. And if she’s happy, what’s the issue?
And what makes you think it is he who is keeping her away? Also, it may be what goes around comes around. You admit that the two of you keep a distance from your parents. Where is the kibud av v’eim here? This may be the just reward from Hashem.
Additionally, you shouldn’t mix into your sister’s shalom bayis issues. If he wants a distance, that’s between him and her.