my sister is really PMSy all the time and has this little mood swings, and is

No, it's definitely not your fault. Some girls can handle their PMS and some girls can't. Sometimes girls just have really bad PMS like your sister where it can be labeled as a disorder such as PMDD.

There isn't much you can do about it, tell her how you feel about it when she is not PMS'ing and see if she will go to the doctor about it because you're very concerned for her well being.

Not only could it be PMS, but she could be having problems elsewhere, and when she is PMS'ing, all her true feelings come out.
 
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