I don't think it's bad, but I'm also sort of equating it to the picture my grandma had up on her wall of her family (so my mum, dad, me, my exP, my DS1 and my brother, along with cousin1 and his ExP, my uncle, aunt and cousin2, and grandma herself)
Since that photo there has been three more great grandkids (cousin1 DD, my DS2, brothers DS. Neither DS2 or cousin1DD made a comment on "why are we not in these photos?" (Bros DS is too young to talk) that I know of, but if they had, they would probably been told "It was something special that we did at the time"
So personally, I say go for the canvas, if you get asked, you get asked, and will be able to put fears out of their mind that you don't want them, you just wanted something special for you/your children.
And you said yourself, you could get another set done with your step-child at some point in the near future.