Basis - joint tenancy property with title transfer and death

Hello,

My parents owns a joint tenancy property with my aunt. When my father passed away, the interest in the property was split 50/50 between my mom and my aunt due to rights of survivorship. 

Now my question is what would be my basis if my aunt removed herself from the deed? We did a deed transfer so the owners of the house is now my mom, me, and my sister.