My main character is adopted when she is 9 and passes away at 16. In the first chapter of the story, she is adopted by a woman who has one teenaged son and grown/married daughters. Because the story covers the span of 7 years, it'd make sense that my character meets the two daughters a few times but I don't have a place for them in my plot. If you were reading my book, would you wonder why you never saw the sisters?
As long as there's an explanation why the daughters aren't around I wouldn't have a problem with it. Maybe the daughters live in another city? or maybe they aren't on good terms with their mom? just some suggestions.
Living in another city doesn't seem to be a reasonable explanation for 7 years of absence. The family is very warm and loving, so not being on good terms with the mother doesn't suit the character of the family.
How come they can't be mentioned like, "one Christmas, when I saw my sisters...." and just leave it to a passing comment.
If you are hell bent on not having them at all involved in the plot (including having them writing letters, or visiting occasionally - which would, for me, help the younger girl to find her place in the world compared to these other woman that are much more fully-formed characters than she would be at that age), then I suppose you could have one living abroad with a husband and children and having no time to visit - they would undoubtedly send letters if they were a warm, nurturing and caring family - and the other sister could be ashamed of herself and not achieving all the things she thought she would by this point. The mother would also lament their absence in such a situation as you have made, which might make the main character feel less loved and other such knock on effects. I suppose my advice would be, think what you want from the main character and how she wants to be defined in this world - and what would define her, in her mind (or the narrators mind). However, a woman who adopts a young girl and already had two fully-grown girls of her own is not one that would be content with never seeing her 'real' daughters for seven years and so they would become part of the plot in that regard anyway. Personally, I would get rid of one of the daughters, have one daughter as having married and moved abroad to find success, and how the mother hears from her occasionally but it isnt enough, and have all her affections go into the MC (perhaps too many). But then again, I tend to have huge character banks, and very rarely would write out a character as I'd rather make my fictional people miserable with a dead son than to remove all evidence of him.
This particular family is one of the most stable and harmonious in the story, so discontentment, longing, embarasment, etc. is completely out of character for them. I do like the idea of letters, though.
Them being stable and harmonious would be affirmed by their relationships; so you would have to make sure that the daughter-mother arrangement was one that was equally satisfactory to both parties involved. Just, having a woman adopt a third girl after having raised two, and then have them not see the mother for seven whole years...well, all the women (and many men) I've met who are in that kind of ballpark would be missing their children like mad. Hell, my own mother gets annoyed if I don't speak to her every couple of weeks - and she is one of many. But yeahhh, for a family or a person to be harmonious and stable, they must have everything they want - or at least have enough that being without some other things seems insignificant. I'm not sure two daughters can be labelled as being insignificant.
Is there some reason you need to have the two other daughters in the first place? If they play no part in your plot, I'd just forget them. These are the kinds of changes you might think about in your story. If you want to show a stable family environment, maybe come up with another way to do it. The mother adopts this girl because she's always wanted a little girl as well as her son and has given up on having one of her own (for medical reasons?)
