I'm going to break our lease because my sister in law moved her cousins in without notifying me
What's the biggest issue you have with them moving in with you?
I'm over crowded and I need more space.
What your feeling is understandable. However, perhaps you should discuss it with your sister in law first? Two wrongs don't make a right, and breaking your contract is a pretty big step that might leave her and her family worse off.
I'm going to let her know my thoughts.
It's great that you are deciding to let her know what your plans are. You may find that it might not be necessary to break your lease.