I have a 4 year old and he has definitely been disciplined for a few years. All age appropriate handling, clear consistant rules and then enforcing those rules. IE we don't hit and we don;t shout. If he does those things I don't tell him off per se but he will get a time out/cool off and then when he has calmed we talk about what happened, he definitely gets it.

I think the hard part will be that the 1.5 year old may test you both a lot, she is probably terrified and will need to know clear boundaries from you. Kids look to adults to make sense of things and it's hard not to react when they push but it is necessary.