It’s possible that your relative got married in a church other than the one they attended. Couples who “run away to get married,” may get married in a church of the same denomination miles from home. They may also get married by a preacher of a denomination whose practices are close to their own. Even couples who don’t run away to get married may not be married by someone of their denomination.
Keep in mind that “wrong” church may have been a church attended by a family member and you just are unaware of the connection. In some families, getting married in the “right church” or the “right denomination” matters, in others it does not.
Don’t assume your immediate family’s practices are what your great-grandparents or other relatives did. Not every family is alike.