I think that informal is great because you get a better view of what the sisters might be like on a day-to-day basis vs. a staged Broadway production. As for everyone being "nice" -- well yes, all of the sisters are going to be nice to you during recruitment, or at least they should be.
What you want to look for are girls that seem like the friends that you have had in high school and college. Not, this girl is nice to me, this house dazzled me, these girls are the campus rock stars, BUT these are the kind of girls I could see myself hanging out with on a regular basis. They remind me of my existing friends. They are like the "girl next door." I could integrate some of these girls into my existing social circle. I could see myself hanging out with them in my PJs and fuzzy slippers watching "The Bachelor" (or your own guilty favorite.) If I get too drunk at a party they will look out for me. If I have a bad breakup I can cry my heart out to them and they will understand.
Do you understand what I'm saying? You'll know these girls when you meet them. You may not recognize at first that this is the chapter for you, but if you open your heart, you'll know it.
Also -- you learn how to decide by making decisions. Making a decision is empowering. As the years wear on, you will get better at it. But now is the time to start.