True, but not everyone has that kind of relationship with their sibling or know the deen. when my sister was married she introduced the guy to me and my brother first. we researched him and then sat with him and talked to him.
If you came to the conclusion this guy, is a good muslim, he is financially stable and you say yes, no problem then it is valid, but if you say no or she does not even ask you and just marries then it is illegitimate marriage