I remember a post where someone was asking if they were TA for taking a 13-year-old to Canada's Wonderland for their birthday with a couple friends but not taking the 8-year-old sibling (who was of course also Autistic). That's absolutely a case where the younger child needs to accept that their older sibling has different friends and interests and is allowed to go do things with those friends - the 13-year-old deserves to go on the rides they want and stay as long as they want, not go home after an hour because Little Sibling had a meltdown waiting in line for Snoopy's Flying Ace Balloon Race. I didn't expect to be included in my sister's birthday parties she had at the house just because I was family and lived there.