My family live a plane ride away so yes, we go together, usually make a holiday out of it. When my Dad was critically ill last year I kept my mum company during hospital visits while my DH made himself useful at home. Cooking, cleaning, shopping, walking the dog etc. basically anything to stop my mum reverting to host when she got home from the hospital. I haven't visited them alone in 6 years.
DH's dad and stepmother live a couple of hours away, if he's in the area for work, he'll stop by for half an hour to say hi alone. If he's going specifically to visit, I'm always invited. I enjoy their company as much as DH's company. When they ask it's always "if you and Chilli are free". There's an overnight event he does with his dad once a year and his step mum always invites me to stay over. I bring a book, she does it for whatever she likes and then we have a nice evening with wine and films.
I'd always be fine with him going to events if I'm ill. The issue OP has is with the people who should be going who decide not to because their partner can't. Would he? Depends on who's there. Small close family dinner, he'd go, big extended family party, he'd cancel as he wouldn't want to be there in the first place. I make the extended family more tolerable.
I wouldn't be bothered if his sibling wanted to meet him alone to discuss their father being ill but I can't see that happening. Supporting serious illness usually involves time and money so they'd want me and his sisters husband there for the discussion. If nothing else, to reassure them that yes, we really would be ok spending X amount for a better care home or extra visits from a carer.