Did you and the relative clearly establish up front that she would go twice a week and no more--or was that your assumption? Is it possible that having the pass means she feels she can visit this person even more often? (Which is a good thing and not to be dismissed because of a misunderstanding). Did you clearly tell your relative that the pass is "charged" every time she goes through any toll gate? I think probably not, and she is completely ignorant that the pass works this way. She likely is assuming that the pass is paid for by some flat, monthly fee and that means she can use it as much as she wants under that one fee. Maybe she simply doesn't know that instead, the pass racks up a new charge every time she goes through a toll gate.
I would see her in person, and say, "I'd like to talk about the pass. I get the monthly bill for it, and it's very high. I'm sorry if I didn't make clear how the pass works. I don't pay one flat fee and that covers all trips. Instead, every single time anyone in your car goes through a toll gate, I get charged, and the charges have been much higher since I gave it to you, which tells me maybe you didn't realize that that is how the pass works. We had talked about your visiting (loved one) twice a week so I had figured on that amount being on the bill. I can afford to give you the pass for the two trips a week to see (loved one) but I can't afford more tolls adding up on the pass. If you're making extra trips to see (loved one) now, and that's all the extra tolls I'm seeing on the bill, that's nice of you, but I can't afford that level of tolls each month. Let's work out a plan where maybe you can get a pass of your own, and I can pay X dollars a month toward that, instead of giving you my pass."
If she is just unaware of how the pass works she may be mortified to realize what she's been doing, and she may indeed have been visiting the sick person more because she though it was "all covered" under one fee that was already paid. Also, she has a loved one who is sick. That would make me scatterbrained and forgetful. It's possible she just isn't thinking about "business" things like tolls and your pass right now, so keep that in mind when you bring it up, and be kind.
And do ask her if the pass is being used when someone else is using her car for other trips, or even if it's been stolen, if she seems upset when you bring it up.
It doesn't have to be a big deal if you are just direct and have a good, calm script in mind of what you will say. Is this person someone who gets defensive if questioned over things? If so, prepare well and be ready to have an offer like the one above -- maybe helping her apply for her own pass and giving her one flat amount each month to help pay her tolls.