Most Children's Hospitals have Sibling Play Rooms (babysitting). Alternatively, if you're doing treatments at an oncology clinic, you might look into R NcDonald housing near a Children's Hospital & switch her treatment/care providers to full time.
When my son was sick I was friends with several 12-18mo cancer families ... Several of whom left the Children's Hospital to drive 5 blocks to the UW hospital to give birth, and then were back 2 days later. Their babies had playpen cribs in the room with their older sibling & at RMcD... As they needed to sleep w Mom for nursing. They took their first steps in the hospital/ we had bday parties for them at RMcD or up in the cafeteria.
Their cancer-sibling was usually schooled in hospital, their other siblings attended the local elementary/mid/highschool for a year.
Many of these families were military, traveling spouse -have to keep working for health insurance, people understand-, or single parent. Where they might have been outpatient with 2 parents a d great support at home... Their families needed the extra support for both the child with cancer, and children without cancer.
As an added benefit, family counselling is onsite. As siblings INVARIABLY are affected (either taking on too much or acting out).
I don't know if that's something that would work in your situation... But its worth looking into.