Honestly, casual cannibalism aside, I found this to be rather heartwarming when I thought about it. I mean if the three were still outside on Gensokyo then Kanako and Suwako probably would've died with Sanae (due to lack of faith), but the move to Gensokyo meant that Sanae would be the only one who dies and the remaining two become lonely. It just warms my heart to see that the three of them no longer hove to be worried about being separated like that, especially after we get a peek of Sanae's life growing up with.