My sister wants to know where to get the HM Rocksmash?

Well you can tell your sister not to worry. You'll find Hm Rocksmash near Mt.ember on island 1. Talk to the old man inside that built the hot springs. He will give you it, also you can heal your Pokemon in the springs. Dont forget, the springs are near mount ember, the entrance is above 2 walking karate guys.
Hope I have helped.

Talk to the proprietor of the Ember Spa.

You can get the HM Rcok Smash in the Ember Spa when you walk to the old man.

Go to the ember spa on One Island and talk to the old man by the water and he will give it to ya.

You must go to One Island and go to the right and you'll see water. Surf north and keep walking north. There are a few trainers. Right after beating two Karate Guys go up and you'll see the entrance to a place named Ember Spa. Go in it and talk to one of the old guys in there and he'll give you HM06 Rock Smash.

Hope it helped!

You get Rock Smash in the hot spring place before Mt. Ember. Talk to one of the guys there he will give it to you. Extra hint: if you stay in the hot water for a while your Pokemon get healed. ze monstar signing off. Roar.

I'm pretty sure you get it from the old man in the hot springs on One Island.

I think you get it at the rock spa in the Sevii Islands.

There is a dude in Ember Spa on Island 1. Talk and recieve.