How to react?

I was in the same boat as you before. I could never do more than pat their back but I've improved a lot. Try understand their situation and how they feel. If you were in their shoes how would you want to be reassured? Sometimes saying that it's going to be okay just doesn't cut it and if there's nothing you can do to help their situation, just show them that you're there for them.

Maybe ask a family member or close friend if you can practice with them?

The problem is that my mother and sister are often fighting and not normal fighting like serious fighting and my sister often ends up crying and I never know what to tell her especially because I don’t want to talk bad about my mother :/