My biggest theory is Jill and Sage are high school friends
It's certainly possible as, iirc, we haven't ever had them together in the same scene yet so a base-line relationship between them hasn't been established in the game as of now.
If they went to high school together, I just hope DPC does
NOT manufacture some drama like Sage being one of Jill's old bullies or some such and then dumping both DIK-Sage and CHICK-Jill MCs in the middle to sort it out. Enough of that stuff.
Let them just be old friends that drifted apart due to life-style choices, maybe even once played together on whatever "classy" instrument Sage's parents wanted her to play (I'm hoping it's the cello, but I digress). Our MC can then bring them back together for a nice
threesome I mean three-piece, yeah three-piece band to play some sweet music together
.