I also get my glasses from Zenni. Yes, you give them the prescription information as part of the ordering process. I've been very happy with the clip-on shades they offer as well, though it takes a drop oof super glue to keep the rubber feet on the clips from escaping.

An important note when ordering glasses online: get your sibling to ask for pupillary distance measurements along with the prescription. They are needed when ordering glasses, and many optometrists will not by default provide these measurements as part of the prescription. If you can't get them from the optometrist you can help take the measurements yourself. I've had a roommate take mine and the glasses came out fine.