Maybe a 2 year old post isn't the most up to date source for information? ;)
https://renryuu.miraheze.org/wiki/Sandra
7. Starts in Havaria port, where you can find the thief Shirinsya from Moriko's events again. Requires the events with Flaire to get the gold armor and weapon recipes first, Sandra needs to be in your party and her previous events must be done, and Moriko's events must be past the choice for her good or bad route in the police HQ.
(Sex scene at the brexeet isles, which unlocks Sandra for marriage.)