Pain is one of those infectious things that doesn't go away when it's distributed. After all, misery loves company. Thus, as selfless and touching as the idea is, I wouldn't recommend accepting her pain. I would, however, recommend this simple enchantment, which will draw from the positive energy of a growing plant to speed up her recovery.
You will need a small house plant, one she can keep in her bedroom. Ferns, ivy, and other herbs do well even in low-light conditions and require only minimal care to flourish. You'll also need a lock of her hair, a small glass bottle with a cork top, a box cutter, a white ribbon, a white candle and chamomile incense.
1. Place a lock of your sister's hair in a small glass bottle with a cork top.
2. Carve your sister's name into the candle. Place it in the open mouth of the glass bottle and light it.
3. Light the chamomile incense.
4. Pass the plant through the incense smoke three times. As you do, recite the following incantation:
Through this plant may her body mend
Her health return, and her sickness end
As it grows and thrives, may she as well
Through loving care and witches spell
Hear now my humble plea
As I will it, so mote it be
5. Place the plant beside the bottle and concentrate your visualizations on your sister's recovery until the candle burns out. Allow the wax to drip over the bottle.
6. Cork the bottle shut.
7. Wrap the white ribbon around the neck of the bottle and hang it from the plant. Leave it tied to the plant as it grows.
Be sure to tend to the plant! If the plant is allowed to wither with the bottle affixed, it is a very bad omen. Administer the same love, care and attention to the plant that you give to your sister (without impeding the latter) and you will see her recovery hasten.