Yesterday Nathan Bransford did a post about absent parents in YA fiction. The consensus opinion, with which I agree, is that the presence of parents interferes with the ability of the child protagonists to go out adventuring (not to mention that you get a built in amount of sympathy due to whatever has removed the parents).
I had a slightly different issue in my novel. All of us who have been studying writing have read numerous times that we must cut down the number of characters to just those that are truly needed for the story. It isn't good to introduce and let readers get to know a dozen characters and even more sub-characters if some of them play little role in the plot or their role could be just as easily done by an already existing character. My problem is that in a standard medieval society, families tend to be very large, often with more than a dozen kids. I do manage to show this in passing by describing some families this way, but when it comes to the families of key characters, I keep their numbers way down. After all, it wouldn't help the readers or the plot any to have to introduce 14 brothers and sisters, even if that is more realistic.
So, I find (for me) unsatisfying but plausible reasons for all of my major families to have only two or three kids. A mother died in childbirth and events have prevented the father from remarrying. A wife who dislikes her husband and avoids her wifely duties as much as possible. Anyhow, I kind of despise doing this, while at the same time I completely understand that having too many family members simply doesn't work for my kind of tale.
Have you had to compromise absolute realism in order to make your story more readable?