A friend of mine in council accommodation entirely reliant on benefits has a sister who is a multimillionaire.
I have been aware of this situation for over a decade, and find it unusual. They seem to be on good terms and visit each other's houses. I know rich sister helped poor sister out once or twice with train fare to family events, but nothing more than that, I don't think.
They come from a fairly average middle class home, father a professional, mother worked part time. Parents remained in their middle class home until father died and mother moved into nursing home.
Both are graduates, both are single mums.
The only time I am aware of it causing any problems are when one cousin ( from council flat) got into trouble at school for "lying" about who her aunty is - she was telling the truth but no one believed her.
I suppose in a truly socially mobile society, this should be common, but I think it is an unusual situation. Perhaps it is more common that I think.
YABU - this is how it should be, and/or this situation is common
YANBU - this is rare.