DS keeps flattening DD!

DS is 3 in August and most of his life he has been a quiet, serious, gentle little boy. Until this year. Suddenly (and yes I know he is 2, but it's getting worse) he has become more violent to those around him (except DH or myself - he doesn't do anything to us). It includes pushing mainly, but also hitting with objects, taking toys, and sitting on his little sister or trying to squash her in to corners or objects.

Everything we try just doesn't seem to get through. And when he smiles and laughs when we catch him in the act it just really makes my heart sink.

His verbal skills are coming along, but they aren't super advanced so trying to have an actual conversation about it is not actually possible. So it has to all come from DH or myself.

Can anyone offer any suggestions/encouragment before we end up having his little sister seriously injured (who is only 1 today and not yet walking) or I snap? I want my lovable gentle boy back.

DS1 (3 in July) sits on DS2 (16 months) lies on him takes things off him, but DS2 is no angel he hits DS1 and takes things of him so I have to keep a close watch on both to see who does what to each other, DS1 responds well to being told he will get sent to bed!! He also likes to push DS2 to a sit (he is just started standing up hopefully will walk soon)

DS1 is a bit of a sookie la la thought DS2 just touches him and he pretends to be pushed over or screams as if he has hurt him.

Not sure if I helped at all but I want you to know you are not alone xoox

Thanks feeb! Frustrating but glad I'm not alone.