<human>: Can you give me the gist of the text in a nutshell?

My sister 21/f has lived for about two years with an engaged couple who I think are 32/m and 26/f from my snooping on Facebook and asking around. They've been engaged since this past fall and seem to be getting married in about a year from now. I'm concerned for her because she kind of acts like he's her boyfriend and his fiancee is her girlfriend or something?

They've been seen in public being really close to each other in every combination you can think of with him/his fiancee, him/my sister, my sister/his fiancee, and I actually saw her kiss his fiancee on the lips when the three of them were getting in their car leaving a restaurant in town around Easter, which is what made me finally decide to post this.

He's way too old for her, and her living with a couple like this is just super weird. Everyone in our family talks about her and how they think he's going to kick her out or something once he gets married. Also, she works basically part time at a retail store and he works doing something at home (she won't tell me anything other than "he works on the computer"), but apparently he's got the money to fix her car when it messes up and pay all of their bills, and that just seems like something he could hold over her head.

I don't really know what to do here because she won't listen to anybody, and I am very concerned for her.
<bot>: My sister 21/f moved in with 32/m and his fiancee 26/f, and they seem to have some kind of weird three-way relationship, but I'm afraid for her.