Sisters Loan Is Coming Back On Me

My sister placed me on her application for a loan to fix her house as a co applicant and she had mentioned that she needed a co-signer and I said I could do it. This was over text as she lives in another state than I do, and then I never heard about it again. Fast forward two years I’m getting calls and late payment dings on my credit report saying I hadn’t paid my loan. I assume I’m screwed but if I never signed anything or received any sort of documents about this is there anything I could do to a get off the loan or take legal action against her. Also is there a difference big enough between a co-signer and a co applicant to have a case?