I also noticed that there isn't a way to increase the relationships of the other crew members besides the 3 other playable characters...I wonder if that's going to be a thing in the sequel...or maybe in a future update?
The next update, version 0.61, will introduce more scenes with other characters in Command Center and technically you can improve the lust of other crew members by playing HoloJack with them and improve relationships by choosing proper answers in their dialogues. Maybe I'll add more options to the conversations for other crew members to give more opportunities for the player to improve the relationship. Which other crew member did you want to improve Kira's relationship with? Riley / Joanna / Thomas / Alonso / Mateo / Szymon / Jonathan?