I know what it is. Her mom once burned a cake she was baking. This obviously must be kept very quiet, and if word gets out, it will ruin shidduchim for all the children.
But seriously, people need to grow up and stop with all this FBI stuff. A person can be a great girl and come from nothing or from a very problematic family and it doesn’t reflect at all one her. Years ago, when people met on their own, they only judged by the person. Or friends would informally say, I know a nice guy or girl who is in school with me, would you like to go out. The answer was yes or no on the spot with no investigations.
Let’s be honest. All this checking out is totally for one reason–Kavod. I want to be able to brag to my friends how well known, prominent or rich the family is. But the gemara says, hizaharu bivnei aniyim shemehem teitzei Torah. Torah comes from the simplest and most sincere families, not from the bigwigs. Yesh nosei isha lshem shomayim.
I wouldn’t let it bother you. Whatever difficulties you have are a badge of honor which you have been able to overcome. There is no shame that your family is not perfect. Another secret: none are. Just some people are better at hiding things than others.