Originally Posted by kiterunner

And yes, I have now looked at a new match who only has a small tree and ancestry is showing me our supposed shared ancestors who are not in that person's tree. Only problem is, it says their 4xg-grandmother was a sister of my 4xg-grandmother, but I haven't got that sister in my tree, and it is a Nottinghamshire family, so I have (previously) looked through the PR baptisms and there is no such sister.

So, proceed with caution! To be fair, ancestry does say "you can view each relationship to see if it's correct", rather than claiming it definitely is.