I had my DNA test done on Ancestry, when I received my results I showed a 1051 CM match to a Sandra. I knew nothing of this person nor could we find any connection. This person lives in Wales. Rumor has it my Uncle was in WWII and he got some gal PG after the war he went back to marry her only to find out she was married. I have been communicating with Sandra trying to figure out how we match. Both her mother and father are deceased. I show no connection to anyone she has listed as her "father" nor her mother. I have crossed compared her 1st cousin's to mine and all of the mothers to both are sister's to my uncle. Is there anyway to prove that she is in fact my 1st Cousin and my Uncle was in fact her dad. She has a living brother but does not want to involve him as she feels it would devastate him. Any suggestions?? She thinks taking a test through another site will give her different results.
I don't know if I understand your statement: " I have crossed compared her 1st cousin's to mine and all of the mothers to both are sister's to my uncle." Are you saying that two of Sandra's first cousins DNA tested at Ancestry and they match two of your first cousins who also DNA tested at Ancestry and they all match at about the 1st cousin level? If so, then that's about as close to proof as you can get.
If not, Sandra seems willing and able to work with you. Ask Sandra who her top 30 or so closest matches are. I'm not expecting 1st cousins here, likely 2nd or further, but maybe you will find someone close, who knows? See if any of them are also matches to you. Maybe that will be enough to identify through which ancestor the connection may be.
If that isn't conclusive but you do have matches in common, load your raw data onto GEDmatch and get Sandra to as well. GEDmatch has a chromosome browser and other tools that will allow you to identify DNA segments that triangulate between you, Sandra and a third person. Those could very well be segments passed down from a common ancestor, and the people who the two of you triangulate with may help identify to ancestor.
GEDmatch will not contain the same people on Ancestry, but will have those who did upload there from Ancestry, FTDNA, 23andMe, and MyHeritage.
Both of you can also transfer your raw data to FTDNA and MyHeritage for very little cost and see what other people you share in common at those testing companies.
Finally, if your uncle has any children or grandchildren, you could ask one of them to test at Ancestry and see if they match Sandra.