You are 13 years old, so just let her go and move on with your life. Try to focus on school and having fun with friends right now. You do not need to be in a relationship at all.
Long Distance Relationships Do Not Always Seem To Work Out The Way You Want It To.
If You Truly Have Feelings For Her Sister I Would Go For It But Wait A While And Find Out If Its Ok With The Person Your Having A Long Distance Relationship With And Then Maybe Take The Relationship With Her Sister Further Hope This Helps
I'm not sure a long distance relationship is what you need at 13. I would let her go and move on at your age. If you decide to date the sister I would be careful. Sometimes it's tricky dating sisters, and can blow up in your face....But that is a risk, and you are the only one who can decide if it is worth taking.