Hi, my wife owns a property abroad in hong kong. Her parents stay in the home for free (no rent). Can i declare it as a property rental and include all the interest?

No ... you do not have a rental ... you have a second home that family lives in ... you can deduct the Real Estate taxes paid on the Sch A .... and the mortgage interest if you qualify ... review the changes here : 

https://ttlc.intuit.com/questions/4482394-how-will-tax-reform-affect-my-2018-federal-tax-return?jump...

could you please clarify and the mortgage interest if you qualify .

Read the mortgage info in the link for the changes.

hello, i want to input the mortgage interest for my second home, where should it be reflexted in the tax return?

Sch A.