I just want to say, Sarah, that this is a beautiful thing to do for your sister. I’ve never been hospitalized due to mental illness, but I have battled depression and anxiety my whole life, deal with intrusive thoughts and a passive suicidal ideation on a regular basis. Anyway, I’m happy to hear that she’s getting the help that she needs to heal.
As for a spell, I had created a spell oil a while ago to help me see myself and move past perceived physical flaws. It could definitely be adapted for your sister’s case to help her see herself as she is without judgement. You can find the oil here → To See Myself Spell Oil 
Beauty Magick ← then I would make a poppet, as someone else suggested, to be a sympathetic representation of your sister. Use the oil on the poppet and thus the oil on your sister.
You could also light a white candle and surround your sister in white healing light. This works very well if you have a good foundation of energy work and can visualize. Otherwise, use a taglock of your sister (her name, a photo, etc.) and place it on your altar with the white candle in a ring of salt.