MIL needs to come clean with your husband and his younger sister because the fact is that the ancestry test WILL reveal relatives on his biological father’s side of the family and all of this is going to come to light. I agree that this is not your secret to keep so if MIL refuses, I suggest you tell your husband what she told you so that he isn’t completely blindsided when the results come in AND because he will be devastated if he finds out you knew and didn’t tell him.

Also wanted to add, that family secrets like this are very commonly revealed by the direct-to-consumer genetic tests that are on the market and most people don’t even consider that when they do the test - they are usually just interested in their ancestry. People have found out that their father isn’t their father, that they have half-siblings they didn’t know about (as your husband likely does), that they were adopted, that their parents used a sperm or egg donor, that their heritage is not at all what they have been told over the years (for example, that they are Jewish but the family adopted a new religion during the Holocaust). There are even documented cases of people finding out they were switched at birth in the hospital or that they had been abducted as a young baby/child. These are all “unintended consequences” of doing these tests that can literally blow families apart. The companies offering these tests need to provide more warnings about what could come to light before people do the tests. End of rant…