With members of someone else's family I would be honest with the adults in the situation and say I didn't like what was happening, and why. When it comes to your own children I personally find a "let them get on with it" attitude decreases the amount of bickering in the long term. Siblings often fight (in my experience of my own childhood and own children) to get attention to ignoring it works best. But if another baby was hitting mine I would remove my child from the situation and if that was not possible, tell the other parents that you do not accept the behaviour and why.