my girl has terminal cancer and I don’t know how to prepare her brother

My cat (Luna (the all grey one) 12F) just got diagnosed with a nasal tumor and only has a few months to live and i really don’t know how my other cat (Blue 10M) is going to react. She’s technically his aunt but he’s known her since birth and has been bonded to her for his entire life. They sleep together every night, groom each other, play with each other, cuddle nonstop, etc. I have a dog too (beagle 5M) that grew up with them but they’re no where as close as my cats are to each other. She had to be separated for a few days after an MRI and Blue stayed outside her door almost the whole time. How do I help him as we get closer to losing her?