I think you should leave her alone.

The very last thing that single people want from our married/partnered friends is having them feel bad for us for being single. And when we are single by choice, please don't try to fix us because we don't feel like something is wrong.

Everyone has a different path in life. Your sister has chosen a different path for the time being. If she's happy, what's wrong with that? She's been through the ringer with relationships, so I'm not surprised that she is taking time to be single right now. And actually its kind of nice being able to fully focus on yourself without constantly having to be concerned with someone else.