My sister's toddler constantly hit my baby.knowing my baby is 2years 10 months and my sister's is 2years and 7 months.how can i stop such an attitude?

Parent the child: Your sister should not allow hitting. When the two play nicely together, let them play. When one hits the other, remove the one that's hitting immediately, then tell him it's not ok to hit. Do not let him return until he behaves. If he does it again, remove him again.

The only solution is: To tell your sis calmly that the kids can't play together until you & she decide how to discipline the boys. If she agrees, since your son may be povoking hers, tell both boys the rule: if you can't play nicely, you'll both be put into time out. As soon as you hear a scuffle, walk each to t.O. Without talking. Read " little people" by ed christopherson for how to do effective time-outs.