There are many insensitive people posting here. And I am talking about those that are blasting the original poster for asking. We all die, eventually. And at that point, no matter how much time or love you share with the deceased, you still need to deal with reality.
My time is not that far off and I would be pissed knowing that my children did not handle things right and paid for crap they did not need, like a cell phone.
Anyways, if Gram is off contract, you can easily cancel that line. If under contract, you will need to call and possibly prove she has passed on.
But, like a car loan or mortgage, you may still be obligated. Part of an issue for the estate.
As to droping to a single line on that account, only you can decide. If one of the remaining two ines is $9.99 a month, it might make sense to keep it vs moving to a new plan.
Also - the unlimited data is tied to a phone number. Not to a person. Not to a phone.