i dont remember who it was talking about, but there is a pasuk which says someone was the sister of someone. we learn from here that one should check out the brothers of a prospective wife.
that being said, rejection of a sibling of a prospective mate is subjective. what i may see as unacceptable may be prefectly ok to you, and vice versa. also, what may be unacceptable for one child in terms of siblings may not be for another. for example, for one child, having a prospective in law who is in college rather than in bais medrash might be fine, but for another child in the same family, only learning in laws are acceptable.