I had a similar situation when I adopted Callie. She had a grown cat with her that was her kitten. At the time I felt I could only take in one cat so I left her son/daughter behind. There was a woman waiting to adopt that cat so I know it got a home but after the fact I regretted I hadn't adopted them both. I'm sure these other cats we didn't keep got good homes and we can think they probably made some one else happy to have them.
This was "my" Mikesch. He was adopted by a family when he was 12 weeks old together with his sisters Lilly and Ella. At this time I thought it would be crazy to keep 5 kittens so I kept Willi and Donna only, but it didn't take long and I knew that it was stupid to give the kittens away but then it was to late........