1-2) You don't actually choose any sister's route, but rather end up with the sister you have the best relationships with. So you can save the game right before the final fight with Istaroth, and then return to the inn. There, you need to give the one sister you want to have an ending with a gift (a flower you obtain at some point). Also, the Bound skill can be used to develop relationships with the sisters.
3) Overall, there are 9 endings: 3 Bad Endings, and 6 Good ones. 4 different ending with each of the sisters, 1 ending with neither of them, and the "True" Good Ending where you go on an adventure with all of them.
Now, Bad Endings are a bit more interesting.
1. If you rape any woman in Pandemonium, you become locked on Evil Endings, and without any extra actions, eventually you'll get Compassionate Demon ending, one where you have never touched any of the sisters.
2. If you rape any of the sisters, you'll get Tearful Music Box ending. I LOVE this ending a lot, so I won't spoil it.
3. Now, if you kill a lot of people with the appropriate command in Pandemonium, you'll get another Bad Ending. It's actually pretty interesting, too, so I won't spoil it either.
4) E.B. has 2 games in addition to Pronant Symphony. One of them is
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, with Seraphina as one of the main characters. The other game is in development, so we can only wait till it hits DLsite store.
