I have two cats, male and female, neither are related, both are now bonded and quite inseparable.
I had two sisters (proper litter sisters, inseparable, slept curled up together, washed each other) but sadly one died quite suddenly and the remaining cat was lonely so I got idiot cat 4 years ago (we've actually just had the 4 year getting you anniversary this weekend). They got on but I think my ginger sister missed her sister.
When my little ginger sister died last year, idiot cat got depressed and hid under my bed for three weeks only coming out for food, not even sitting in my meetings(I was on the verge of calling the vets I was that worried). I started looking at rescues with no luck, came across this little cat who they said was suitable to go to a home with another cat as long as they were a sociable friendly older cat. It works because neither are dominant cats and both are idiots.
I always fail with the slow introductions, so these two met on day 2.
Idiot cat is 12, his sister 6. She matches his playfulness. When she went missing last December, he spent the 6 weeks moping around just staring out the window. When I came back with her after trapping her she immediately went over to him and nuzzled him, she walked straight past me. He calls for her when he wants to play, she does the crab dance when she wants to play.
It's their personalities that matter. Some cats are meant to be only cats some need company.