How To Act If Your Child Is Bullying Their Younger Sibling

My child is bullying their younger sibling, what should I do?

This is flagging up something that is not working in the way your child wants it to work. It might be in terms of how much time and attention they're receiving relative to their siblings. It might be flagging up some feeling of displacement to older children, which is not uncommon for them to experience. Either way, you need to find out what's driving that behavior, and what they're getting out of it, and very quickly put something in place, and communicate that this is not acceptable on any level.