How do I?

My in-laws' are listed on title & mortage of our property, at request of sellers, who carried back the note. I have documentation of all money being paid by me, and no interest deductions have been claimed concerning this mortage previously- How can I claim this deduction if I am not on title?

In order to deduct interest payments on your residence, you must be legally liable to pay the loan. If you are also listed on the mortgage, then you would be legally liable for the loan. If you are not named on the mortgage and the deed, perhaps you can get the lender to add your name to the mortgage and get your relatives to put you on the title.

Would it be in my intrest to have an atty. look over the prommissory note, The parties holding interest in the note are the surviving children (grown) of the seller. Could there be an " acceleration trigger" if title & mortage change?

It is possible that there is such a clause in the mortgage. If you are unsure about that, I definitely advise you to see an attorney about it.