If your are living with a Mortal sin, then I’d say you need to ask for forgiveness and “repent from doing” whatever the Mortal Sin is. If you are stoping whatever the Mortal Sin is and are asking the Saint to pray for you, I don’t see why the Saint would ignore your request.
If you are continueing to live in Mortal Sin, the Saint may still choose to pray for you that you will Stop living in your Mortal Sin, but truely I would think a person needs to have a repentfull heart and turn away from the Mortal Sin they are living in.
These are just my thoughts, I can’t truely answer for the Saint, but like I said, these are the thoughts that come to me.
I will also pray for you.