I grew up in a small town that did not have a high school. There was one smaller school in a neighboring town that took a very limited amount of students from my town (we had to apply, and grades and discipline records factored in), there were 2 or 3 religious high schools available, and one huge city school that accepted any students from our town who enrolled.
I was active in a church youth group, and even though most of us lived within walking distance of each other, we represented about 5 or 6 schools. It was fun competition, and it didn't interfere with our friendships. We were really loyal to our respective schools, but our interaction with each other reached far beyond school spirit and whose football team was better.
If you encourage and nurture good family relationships, family meal time, family activities, and a healthy and respectful attitude in the home (starting with your husband and yourself), then your girls will most likely keep their good relationship with each other. After all, in 4 short years they'll probably be going to different colleges, moving to different cities, etc. It's the solid foundation that you build within them that lasts and that is most important, not their alma maters.
Send them to the school that is best for each one of them. My neighbor had 3 kids in 3 high schools (she adopted them and they all had emotional or physical or intellectual needs that required things that different schools offered or excelled in) and yeah, she went a little nuts on parent-teacher days, but she kept a great organizational chart and she knew she was providing what her kids needed.