First of all, Lana is not the saintess, but a descendant of a goddess and that's why she has that healing ability. Second, Shin and his brother are actually royalty from another country. Third, Lana is not the only reincarnator from modern Japan but also Shin, who happens to be the former love interest of Lana in her previous life (both died in the same accident when they were going to meet, he died while trying to protect her, but she didn't know until he reveals it). Finally, both also had reincarnated within that world, not Japan, in a previous era. It hasn't been mentioned in the manga and it's way ahead within the story, I believe after the issue with Sandra is dealt with, supposing it follows the WN.