It seems to me there are simply way, way too many variables for strangers on the internet to be able to make any useful advice. Like:
How old is the OP and his father?
What does your sister think of this? (I’m assuming your father asked her to cosign, too.)
How is the OP himself getting along? How comfortably off is the OP? Is he married, does his spouse have a good income – and how does s/he feel about this? Children? Does he have his own house/mortgage or will need his credit to buy soonish? Can he pay off the new loan PLUS the demands of his own life if need be?
Is his father still employed and in decent health, as in, has the capability to pay off a new loan? Or is the OP likely to need to help cover expenses in assisted living?
Is this house in good condition and a good location, as in, likely to be able to sell quickly if necessary?
What is going to happen to father if you don’t sign? Can he pay his regular bills? Would he end up having to move in with you or end up in the streets?
On and on.
Note: I don’t expect you to answer any of this, just things that could majorly affect what the ‘right’ decision for you is.
It seems to me that a consultation with a lawyer is step one. What really ARE the legal implications of being on that deed and/or cosigning. Get hold of copies of the paperwork to take with you.
And if that don’t shut the door already, then find a financial consultant.
Yeah, they’ll cost you some time and money, but you need good, professional advice before you find yourself knee-deep in the alligators if they’re lurking.