I don't want my cousin to use my deceased mom's phone
Why wouldn't you like your cousin to use her phone?
The phone is a cherished object to me.
I'm sorry for your loss, she is your mom and you are were the closest to her. You have all the right to not share anything of her to anyone else when you are healing. Her things are now very precious to you so take good care of it since those are the last things she has.
Yea I am going to try and keep it safe. This is a memento that we need to hold on to.
That is understandable. Tell your cousin your reasoning, I am sure your cousin will understand how you feel.