I have 18 months old twin girls who recently started to bite, both their sister and us. It's not like when they were teething, it's an aggressive move.
We've tried to talk to them explaining that it's bad and it hurts, and making them apologise. Sometimes we can see that they feel bad about it, but they will do it again.
What can we do to have them stop biting?
Well, we've tried something new:
When they're about to bite, we tell them to stop and to make a kiss instead.
It seems to work pretty well, we'll see how this goes.
First, try to figure out the feelings going on that are resulting in aggressive behavior. Work on resolving that and you will go a long way to solving the issue of biting.
Second, when you're in the moment and need an immediate solution, place all the focus on what you do want instead of what you don't want. So "give me a hug (or kiss)" or "give each other hugs (or kisses)" instead of "don't bite" will always work much better.
When it doesn't work, just keep them separated or whatever is needed to keep everyone safe. Explain, gently, why you need to keep everyone safe. But avoid scolding or shaming as that will make them feel worse (and in turn, potentially instigate more aggressive behavior).
Finally, of course never hit or spank or bite them in any form of punishment or for any reason. That just teaches that physical aggression is a way of solving problems, which is something they could eventually model themselves. Hope that helps.
