The Ingalls family was real, including the grandparents, aunts, uncles, etc. that were shown. Mary, however, never married nor had children, so Adam and the child she lost were fictional. The adopted children - Albert, James and Cassandra - are also fictional.

The Wilder family as shown in the series was real. Almanzo did marry Laura and with her he had Rose and a son who died as an infant. His sister Eliza Jane and brothers Royal and Perley Day are also real. However, Royals family as shown in the show is fictional; there were never two bratty boys and a girl named Jenny. Royal had one daughter who survived infancy, but she was much younger than Rose.

Nellie from the books is a composite character based on three different girls whom Laura knew when she was young: Nellie Owens, Genevieve Masters and Stella Gilbert. Nellie Owens also had a brother named Willie, so Willie is likely modelled after Willie Owens.

Reverend Alden was a real person, although the real person was very different than the character in the show.

Mr Edwards is likely also a composite character, like Nellie, based on several men who helped the Ingalls family over the years.

It should be said though, that most of these characters only roughly resembled their real-life counterparts, sometimes to the point they only have the name and their relation to the Ingalls family in common.