Spring,
6 month arrears and with an adverse lender. No lender high street/sub prime lender will touch you for the forseeable future. Plus as your sister is joint owner/mortgage holder she will have to agree to any change of lender or anything to do with the property. Also when you took the mortgage out you are both fully responsible for maintaining the contractual mortgage payments including the other persons share if they do not contribute.
If you and your sister are still speaking I would imagine best option would be to agree to sell up, split any equity and go your separate ways!