In truth, as long as she is eating an appropriately limited amount of good quality rabbit pellets and she is hydrating well with water, she doesn’t actually need veggies — pellets do the vast majority of the work in filling in the nutritional gaps a mainly-hay diet leaves.

To ensure you are feeding good quality pellets, see here: https://wabbitwiki.com/wiki/Pellets

Re: the variety of veggies, honestly, I tend towards the “don’t over-engineer things” angle, especially since again, pellets are doing the nutritional work. If her sister loves those veggies then you can continue offering them and if she eats them with her, great, if not, well, she has pellets so nutritionally she’s fine. Some rabbits are slower eaters and just generally wary of new foods (my Fable always knows when we start a new head of romaine, even within the same bag), so leaving the plate out can be beneficial. The veggies do wilt, but some rabbits will indeed eat them if you give the time and space. If the sisters are properly bonded (i.e., both spayed and went through bonding) so they are sharing plates, you can try doing two plates so the sister doesn’t eat up the veg, or wait until they are sleeping and sneakily place a plate by the picky-veggie sister and hope she eats it before the sister notices.

If she is eating hay and pellets, it’s unlikely she has dental pain. Hay is the most coarse and chewing-intensive food option of the 3, therefore, eating hay is a great indicator that the teeth are likely OK. That being said, presentation can matter, so you can try cutting the veggies into narrow and short strips for easier eating. My Wick cannot eat veggies if they are not cut up small and thin due to his teeth alignment.

If you want to provide extra flavor options and she won’t eat fresh, you could try some dried herbs. I have one who can’t eat fresh anything due to digestive trouble and he eats dried sage, mint, dandelion, chamomile, etc. No guarantee she would like it though if she’s picky!