Just use the name.

I'd sit down with your cousin and explain that you've had that name in mind for a long time, that it saddens you for her, but that you really want to give this name to your baby. See how she reacts. If she doesn't support it, think carefully about whether using it as a second name could be an option. I assume you're in good/regular contact with your cousin, since you know about the miscarriage and the name she chose. If, however, you don't have much or any contact with her, I wouldn't consult her, but would politely inform her in person before the birth that you're going to use that name.

Hi, really tough.
I’d definitely talk it over with your cousin.
Not sure what I’d do myself.

Depending on your contact with your cousin, I'd make the choice to do it or not. And yeah, make it discussable... it's a tricky issue... it's also a bit sad for your cousin with the miscarriage.

I wouldn't do it myself, I think. But after discussing it with your cousin, it could work in my opinion. The second name is a different story though. I'd also prefer to choose a different name in this case, to avoid associations, and because I would have done the same with a live-born baby. But if your cousin is okay with it, it's fine of course.