More and more I hear people telling me,
"The dealership applied a ceramic coating"
I say, tell them to PROVE IT.
Make them show you the bottle the product came in and see exactly what it is they applied.
I'm not saying I don't believe that dealerships are not applying ceramic coatings, it is the trend for the future. But you get what you pay for and it takes time, work, the proper tools and the proper towels to correctly apply a ceramic paint coating and historically dealerships couldn't apply a coat of wax without screwing it up. So how are they all of a sudden now able to apply a ceramic paint coating?
Trust but verify.
As to your question....
First - some ceramic coatings leave the paint feeling rubbery. Personally I don't like coatings that leave the paint feeling rubbery and I won't use them. I prefer the paint to feel slippery and I know from experience this makes drying safer and faster because your drying chamois will slide over the paint instead of stick to it.
As to whether they applied a coating or not and whether or not you can apply a sealant over whatever they did, the answer is "yes". Go for it.
If they did apply a coating Wolfgang Deep Gloss Paint Sealant 3.0 doesn't contain any cleaners so all it's going to do is sit on top of the coating, not remove it.
If they didn't apply a coating then you're doing your sister-in-law a favor. (nice guy)