Building on the earlier suggestion, I love the 85mm!

• Canon RF 85mm f2 Macro IS STM: Incredible bokeh.
• Canon RF 50mm f1.8 STM: Great value. The 85mm is definitely the winner, dont skip it!

Regarding what #1 said about "An 85mm works well and Im happy with..." - they hit the nail on the head. Using a 35mm for close-up portraits is risky business. You end up with that big nose effect because of the perspective distortion, and your sister probably wont thank you for that in her graduation photos. It basically stretches the features closest to the glass. If you want a reliable, middle-of-the-road option that handles indoors better than the 85mm, look at the Canon RF 50mm f/1.8 STM. Its tiny, super affordable, and basically the standard for a reason. It wont give you as much creamy background blur as the Canon RF 85mm f/2 Macro IS STM, but its way easier to use in a crowd without having to stand 15 feet away. A couple quick tips:

• Distance is your friend. If you use the 85mm, make sure you actually have room to back up in the venue.
• Watch your shutter speed. Even with the R8's great sensor, shooting indoors can get blurry if you drop below 1/125s. Tbh, the 85mm is the correct portrait lens technically speaking because of how it compresses the face, but the 50mm is the safer bet if you're worried about tight spaces and staying under budget. TL;DR: Grab the 85mm if you have the space to move, but the 50mm is the safest all-rounder for a graduation day.

• An 85mm works well and Im happy with it, no complaints.
• Careful with 35mm tho; itll distort her face if you get too close for portraits.