This is totally normal. Take one day a week and get lost somewhere-- literally. Take a day trip, go jogging, read a book, surf the internet and plan some exotic trip that you will take when you graduate in 4 years! Just lose yourself in your thoughts.
If you can, plan a day trip or a road trip. Sometimes a change of scenery will do wonders.
Watch your time management. It is easy to get burned out, especially as you get closer to finals. However, you may just be spending too much time with the same people and the same drama. Think about getting involved with another campus activity in addition to Greek Life. Something that is all your own and separate from your sisters. That saved my sanity many times!
It will be ok! Don't be afraid to rely on a trusted friend or to speak with a counselor. Almost every freshman I know goes through a phase of the blues in the first semester. There are a lot of new things happening in your life!
Remember to watch your health and take a multi-vitamin each day. A lot of times, what we are eating and drinking can have an effect on our moods. The same goes for going outside and getting some sunshine! When it gets colder, there are quite a lot of people who go through mood changes because they are lacking in Vitamin D!
Hope this helps. It will get better. Finals will be before you know it and then you have all Winter Break to catch up with your HS friends and compare notes on your first semester away from home