If you are looking to rent to build your credit score before you apply for a mortgage there are two things you should know. First, rent payments, including payments to a relative, usually do not automatically appear on your credit report so they do not help build your credit score. There are a number of relatively new services such as Rental Kharma and RentReporters that you can sign up for to have your rent payments added to your credit score. These services require you to pay a one-time upfront fee of $40 to $60 and an ongoing monthly fee of $10 - $15 to report your rent payments to the credit bureaus which can have a positive impact on your credit score. These services may require the participation of your landlord, in this case your relative, and you can contact the services to learn more about their requirements. You may also want to contact the three main credit bureaus Experian, Equifax and TransUnion to understand if they offer a rent payment history product.
The second point you should be aware of is that some mortgage lenders may require you to verify rent payments that you have made over the past one to two years before you apply for a mortgage, especially if you are making payments to a relative. In this situation it is helpful to pay your rent by check or automatic bank deduction so that you can maintain a record of making the payments, such as with bank statements or cancelled checks. This way you can provide to the lender verification of your past rent payments when you apply for a mortgage.
Including your rent payments on your credit report and having a record that verifies your past rent payments should help you build your credit score and improve your ability to qualify for a mortgage in the future.