oda already consider other sh to be a family,
you saying if blood related they can't join..
clearly bonney is being shown by oda to be alone/no crew and only her father, then we get kuma back story of suffering and having to endure all the pain till luffy save him from his fate as a slave.
also it would mean sh get new father figure in the already sh family, and little sister
also he protected them, without them knowing, now it there turn.