Can one make out a will on a property you have inherited while you are still in the process of doing the transfer. Or should the transfer process be first completed to do so. Please advise.

I am an only child and both my parents are deceased. They were married in Community of property. While they were still alive they had a mutual will drawn up that stated if one of them dies then the surviving spouse inherits the property and when the surviving spouse dies then I inherit the property. When my dad died my mom did not transfer the property into her name as she was not financially able to do so. When my mom died I fully inherited the property as well as all the assets in it. I am now in a position where I have to pay the transfer from my dad into my mom's name and then I must still pay it onto my name as well which is a huge amount. I always thought I could just pay one transfer from my dad into my name but was told it cannot be done like that. Please advise as to what my options are in this matter.