You know them and you felt the vibe, so I am not going to argue with your “guilt” (or, earlier, “pity”) read on their motives. But for what it’s worth, when I read your first post, I was surprised by that. To me, it just sounded like a super idea. I was picturing your parents’ wall with a large photo of the whole group surrounded by smaller frames of the other shots, and how I-wish-I’d-thought-of-it cute it would be to have that shot, not that it would look like some fake family unit as a consolation. Maybe something more like that was what they were thinking, too.
For sure it wouldn’t have occurred to me that they were saying a photo with your niblings are the best representation of your lifestyle. (I bet at least some of your siblings and their spouses would have the same objection, for that matter.) Family photos aren’t, in my experience, about trying to represent anyone’s lifestyle anyway.
Anyway, of course you don’t have to do it if it makes you uncomfortable notwithstanding motive. I like the teapot idea, too, a lot, if you don’t mind packing one up.