For those baffled by the game, there's basically three sections to the game.
1) Use the normal girl and save something like 15-16 girls. There's a little radar ping when you get close to one. You want to get the four tools, and all checkpoints. Pickaxe breaks rocks, Salt breaks the eggs, Perfume(?) deals with the bug piles, and the Bombs destroy cardboard boxes. After that....
2) Die or just hold Q to switch to the sister. There's like 7 more girls to save - but since she can't break through obstacles, she has to use the minimap to teleport to each of the aforementioned checkpoints, and find all the girls in the area. If the radar pings, search more in that area. After getting all of them, you are able to go past the random in-game barrier in the bottom right part of the map. Talk to the girl there, then you'll be able to break green rocks.
3) At this point, teleport to the church, break through the green rocks, then checkpoint and save all the girls in that new area. Switch back to the first girl and clear out the girls in the new area as well.
Not super detailed, but a little brute forcing map/mechanics never hurt anybody. The switch to (2) was the hard part since I played on the "can't get preggered" difficulty and hence didn't realize I had a second character (who can somehow see girls that first girl can't)