I suppose it does in some way. Having siblings teaches you how to share and compromise, since it's not about you all the time. There's someone else to take into account other than yourself. Having siblings is, in my opinion, very helpful and beneficial when it comes to how you act socially, so why shouldn't that also include relationships?

I guess it's not something conscious, but rather subconscious since having siblings just help shape your personality, just like being an only child shapes your personality in a different way.