People use the expression “marrying into the family.” It is true that family is an important aspect of marriage. But ultimately you are contemplating marrying an individual. You may marry “into” a family, but you are not marrying “the” family. If you have an issue with a member of someone’s family, that is generally not an automatic reason to reject dating someone.
Having this challenging person in your spouses’ family may complicate matters somewhat, but it should not be a primary consideration. A prospective date should be judged on his or her own merit, and not shrunk down to the size of his or her most unappealing relative.