Hitting

Hi mommies . I am a full time working mom with two kids . So my son recently turned 20 months and has started hitting other kids !😔😔 I recently had a baby girl who is now 2 months old .. also my in-laws were living with us for the last 8 months and they pampered him no end .. but after my delivery they left and I had to put my son in daycare . So basically he had some major changes in his life in past two months .. n I guess that’s the reason he has become aggressive and hits other kids .. he sometimes hits his little sister also but other times he loves her .. please a little help here . Any advice how to deal with this .. any other mom in this situation ??.. I am at my wits end at what to do !!