Hi @kirsty4 
I’m not sure of any particular spell, but I do have a few links I could share that may be helpful.
→ 
The Art of Healing - Sending Energy to Others
→ How To Send Healing Energy To Someone – Spells8
→ Magical Healing Salve – Spells8
→ Healing Spell for Someone - Q&A - The Spells8 Forum
I’m sorry to hear that you and your family are going through a rough time with this. I can’t imagine the effort it takes (from everyone) to support someone important in your life that has cancer. I’m glad to hear that you’re there for her in whatever way you can be. I’m sure just having that support is extra helpful for your mother-in-law 