Catfishing
Elder siblings often feel a strong sense of responsibility toward their younger brothers or sisters. They’ve grown up being told by their parents to look after them, and this is hard to shake off. It is possible to overstep the mark, and hard to let them make their own mistakes when they get older.
This brother must have thought that it was his responsibility to keep an eye on his sister’s love life, and didn’t approve of the way it was going I think that it was totally unacceptable to go as far as he did, though. I wouldn’t be surprised if his sister feels embarrassed and totally betrayed.