What if a revert sister does not have any Muslim relatives?

Generally, it will be the imam of the masjid or somebody in a leadership position in the community who will take responsibility for vetting the people that come to her.

Will not simply be there to hand her off to any random guy that comes around, but it'll be a person who actually takes to heart the fact that if something goes bad in her marriage, he's going to have to step in the future.