"I'm not a homophobe, but it's not okay for someone in my family to be in such a relationship for X, Y, and Z reasons, we have standards you know"
Ugh... too real.
Not saying that a sibling shouldn't get upset about the huge age-gap here, though. That part's totally valid. As a sibling, exaggerated raising of eyebrows at the slightly questionable parts of your siblings' relationship choices is, like, practically part of the job description.