Hello and welcome to e health forum.
This is a very common situation, in most families. You need not bother about such issues.
The best way, to handle the situation, is to have a open and clear conversation about the issues that are bothering her, and the steps that can be taken, to help her out.
In most of the cases, where the individual has a normal healthy stress free mind, the situation might be solved amicably.
But in case, your sister might be having issues like depression, mood disorders, etc she might benefit by consultation with a psychiatrist who can help her deal with her issues as well as treat her with medications if needed.
I hope this helps.