You can actually, there's a harem path as well, where they're all together.
How could i get that route? I was involved with two of the sisters than got forced to break up with one of them
It varies per girl, but in general you should be nice and truthful to them. Most of the important decisions regarding the start relationships are at the beginning of the game, with later choices affecting established relationships.
Don't sleep with older sister after going bowling to keep both sisters affection.
