I'm fortunate that this has not be an issue. My family likes my husband & his family likes me. As long as the SO treats the other with respect, kindness, human decency. If others have a problem with the SO anyways, they are the ones who have the issue.
I've been told that before and for my past girlfriends they were all right. Though to be told that at the time was a bit annoying. Mainly because most of them had no idea whether it was true when they said it and guarantee you that they didn't. Though ended up being true.