Working hard at home on his speech at-home program will help. I hope your sister is observing the speech lessons with the therapist so that she can do it. Twice a day speech work - twice a day. It's THAT important.
The therapist will probably teach his "ep me". (That's "help me".) When he learns to say it and why to say it, it will help him be much less frustrated. While he is learning to use words, when he starts fighting and losing his patience, she should get face to face with him and say "Use your words. Tell mommy what you want." Maybe he'll say "ep me". If she knows what it is he wants and can help him, it will make a big difference. If she doesn't know, she can then say "show me".
It's important that he feels that he is being heard. It is also important to stop him from hitting others. She should take his hands into her hands and hold them tight so that he cannot get away - he won't like it - and tell him "We don't hit mommy. Hands are for helping, not hurting." And she should hold onto him until he listens to her words. Then she can let go. If he hits again, she should do it again for a little longer. If he starts having a tantrum, she should put him in a playpen and walk out the door so that he cannot see her. Time out probably won't help this young - I'd restrain him in the playpen instead.
The reason he doesn't do any of this in daycare, D., is because they consistently give consequences for poor behavior. He doesn't feel like he has boundaries at home. That's the problem.
Hope this helps your sister.
Dawn