If I borrowed money from a family member for my mortgage, can I claim a mortgage interest deduction?

You can claim a deduction for interest you paid to anyone but the person who received the interest needs to pay taxes on the interest received.  You can ask your relative to issue you a 1098 for the mortgage interest that you paid and then they can enter it on their taxes as income as well.

@monica-lincoln 

please clarify what took place.  to be Qualified Residence Interest 

1) the debt must be acquisition debt limited to $750,000 for post 12/15/17 mortgages

2) the debt must be secured by the home

3) if a refinancing and the other criteria are met only the balance refinanced qualifies unless any excess was spent to make substantial improvements.

these rules do not apply if the mortgage was for rental real estate.