What do I do if she is in an unhappy state of mind?

I have a sister at home who is younger than me. Been with her husband for 7 years. She just turned 21. They got married at 20. Young. Her husband has been a good guy in our family. They get along very well. This year, maybe last.. Her husband has been acting differently. He has some form of mental issues, and I know we're all not perfect here. He has changed character on our family. He IS getting the help he needs. I'm not sure how bad this all is..
He has started getting mean towards my sister. This has been going on for about a year now. Could be longer behind closed doors. Every couple fights, I get that.. My sister has been scaring me lately. We were walking in Target. She was telling me how bored she is in the marriage. How everything just sucks. That she ended up making out with a guy she found to be attractive. She felt somewhat guilty, yet not really. THIS IS NONE OF MY BUSINESS TO JUDGE.

I just stood there, and I got scared for her really. I personally live on my own, and she's already faced bankruptcy. I'm in a good financial state. I have my stuff together. 23 and single. I just compared myself to her because I don't want to ruin what I got. She is my sister at the end of the day.. She has a dog who is somehow aggressive. Doesn't like other animals. I plan on bringing another cat home. I plan on moving least 25-30 minutes outside of my home town.

I'm just freaking out for her, and I don't know what to do!
What do I do if she is in an unhappy state of mind?
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