It MAY be a problem if sisters or mothers and daughters "fall in love," but they are so close anyway, how could you tell, except if they bond so tightly together there's no room for friendships and romances other than the filial ones. That'd be amazing though, and not ALL bad.
I get an amazing one just watching them get so much pleasure.. wish as a guy I cold have multiple. I think this is what guys could learn by watcvhing esbans using their tongius and mouth to help their Bi-Ladies have multiple orgasms before they have their one at the end with their mate.
It's that casual attitude about lesbian sex many women love; none of the heavy emotional baggage, just really great friends having amazing orgasms together.