I have two metamours that have been in my life for 2-3 years. My girlfriends husband I get along with well enough and we have a healthy repsect for one another and come together when need be in support of our shared love. We don't really hang out except if we are at the same function. We don't have a ton in common and would likely not be close friends but we work on making sure we communicate when necessary and laugh and joke around when we are in each others company. To me its just what I expect and feel comfortable with. I have my own relationship with his wife and he is ever present, but not over bearing. It works for me.
My other metamour I don't see unless our families go camping. I send her funny links to videos, buy the kids gifts for Christmas and birthdays and sometimes arrange play dates for our kids. She never reaches out to me, doesn't want to spend time with me, disagrees with a lot of what I believe in, talks through her husband a lot of the time and if there was ever an emergency I would likely not hear about it from her. It breaks my heart, but I keep doing what I am doing in the hopes of change. I don't want to be her best friend, but I do want to be considered and our kids to be thought of as they are friends. I think she doesn't think about me at all and prefers it that way. Don't ask don't tell kind of thing maybe? I'm not sure.
PN has had loves come and go and I quite often don't really want to know them much. They tend to be a lot like me and while that is kinda cute at first, all the things I struggle with about myself I find come up for me. I try and just be friendly, considerate, give them space and remain present if I am needed in some way.
I don't think metamours have to be long lost sibling and best friends. It think that you would do best to keep the boundary of not hanging out at their house and enjoy your partner when you are with him. It sounds like you two are doing great and might even be great for communal living some day but the work you will need to do is with her. The dream of communal living is a nice one, but it won't work if your relationship doesn't bloom with his wife. I wouldn't invest too much into that for a few years and just have a good time with what you have.
There are some threads here on "metamours" if you are interested in going to the search engine and looking at the tag of that name. Maybe even under "metamour." Maybe you will find something that is helpful.
Anyone want to be friends on Facebook?
Send me your name via PM