Castle's Mum and Dad had been trying to sell their home for a couple of years. Their plan was to travel and do bucket list things. My guess is that Sis's cancer diagnosis changed their plans and they decided to stay here to be close to her, which is obviously understandable. If they are happy living in her garage to be close to their family, then why not. Their child, their baby girl, got diagnosed with cancer, for any parent that is a living nightmare.
Whether the private treatment was because of Sis's husband's employment or her parents paid, is neither here nor there. If you can go private then why not. It also takes the burden off the NHS.
My Dad had, and died, from cancer, the NHS were fantastic, from the treatments themselves, including experimental one's, to the staff, was brilliant. He ended up needing open heart surgery, a nurse who worked on the ward finished her night shift and then spent the next 8 hours in the waiting room with us because "I wouldn't be able to do much else because i'd just be worrying about how he is". A celebrity was being treated there at the same time as my Dad by the NHS and recovered, they were really nice.
Castle's Dad earned his money through hard graft. Castle got very lucky in a very short space of time and got a lot of money, her 'earnings' have now decreased substantially and will continue to do so. Her siblings made hardly anything and continue to make not a lot. Sis obviously will have needed to lessen up on her bizniz during her illness and treatment. Unfortunately, unlike a real job, there is no sick pay. If you aren't hustling, you aren't getting paid. The money they have been given by their parents will soon be spent, I wouldn't be surprised if a large chunk goes in propping up their bizniz's. It's a bit like when lottery winners end up totally broke years later because they just had a huge amount of money given to them which they didn't earn so get through it like crazy.