What does “socially acceptable” or “whatever makes you comfortable” have to do with taava? It doesn’t, and these are halachic matters, not whatever you think is for you.
Writer Soul, it is understandable if a younger male cousin (mistakenly, but still) believes you’re like a sister to him, more than one who is closer in age to you.
ZDad, thinking of a cousin as a brother or sister is a nice concept – until somehow one of them doesn’t any more think of the other as only a cousin when something about the other person appeals to them in a non-cousin type way.
Chazal knew a lot better than some people give them credit for. Ask a Rav, as I wrote above.