Could I be a secondary?

About this time last year my friend G and I were talking in the college cafeteria and I told her about how I felt trapped in my house with my parents using me as a 24 hour babysitting service which would be understandable if they had to work all the time but this is not the case. They go out with friends or just out for a drive or they go to take care of an errand that just one of them can easily handle alone. With all this I have no life outside of my siblings. Having to babysit all the time when my mom was in nursing school was fine. But I just wanted to be free to live my own life now. G (the sweetie) offered me to go live with her and her boyfriend when they buy their first house. I told her that was sweet but I cannot move into a soon to be married couple’s home. Then she told me that she was not just offering to share their home but their bed too. I froze both scared and excited. I am bi and have always found G attractive but never imagined this kind of relationship. I wanted to say yes but I am far too insecure about my weight to consider such a proposal. I told G this and she informed me her fiancée liked big girls so it’s not a problem. I told her it is a problem for me and I can’t even consider a normal relationship with someone until I get not only my weight but my life and emotional well-being in order and could she please give me time. She agreed.

 A couple months later I was eating at a Vietnamese restaurant when G and her fiancée walked in and said hi. This was the first time I had met him and was a little nervous. He was a gentleman at all times even though I knew G and he had to have discussed her offer to me. None of us mentioned anything about the offer though because it seemed as though G had just had another disagreement with her family and was feeling very hurt at the moment. Her fiancée was holding her hand and speaking soothingly to her. I longed to reach out and comfort her too but was not sure what I could do or what was allowed to do in this situation. So all I did was tell her if she wanted to talk I would listen but she did not so I didn’t press. Since then a few months after that I was once again "proposed" to by G. She is truly serious over this matter. I have done research to try to make sense of this situation. I discovered the subject of polyamory and founded that in this relationship I would be a called a "secondary" to both G and her fiancée. I am still having problems with my weight and my family living situation but I am making strides to break free of both. I really do want to consider G’s offer but am unsure if the situation will work out at all. I feel as if I need to examine every situation that could possibly happen between all of us. This is not even mentioning the threat of jealousy. When I think of it, I believe I would be very happy as a secondary to both of them. However I do not know if they will end up regretting it and that is what I fear the most.