I think you should set it free. It does not sound as though you are properly prepared to take on the care of this frog and if that is so, it will likely die.

Frogs require certain housing, temps, humidity. You have to dechlorinate all of their water. You have to keep their homes clean and change the water in their water bowls every day to two days. You have to mist twice a day to keep their homes moist. At minimal you need a clean tank, screen lid, paper towel for their floor (misted with dechlorinated water), or coconut fiber or plantation soil substrate (moistened with dechlor water), a shallow water bowl, heat source when it's cold, amphibian calcium with vitamin D 3 powder, amphibian multivitamin powder, live food (crickets, small roaches, cut worms, etc.), safe plants, etc.

It's takes some effort to get things set up.

If you are willing to do it all?

I know frogs are cute . We here all love frogs. That's why we are here. They are just trying to give you logical advice.

Your sister will understand if you tell her it could die.

It is best to get set up first, with the proper set up for the specific species. Then purchase your frog(s).

Try reading through the article here that helps to see which frog(s) is best for you. They can vary quite a bit in their housing. There are also differences in their behaviors. Some are more active than others. Some only wake at night, which would be boring for her to watch if they are always sleeping. Just things like that.

Any thoughts?