Good evening. Girls, can you tell me at what age did your kids start t…

Good evening. Girls, can you tell me at what age did your kids start to sit? My first son sat at 7.5 months, and by 8.5 months he was already standing. My youngest is going to be 8 months in a few days, and he just crawls everywhere like a bullet but has no plans to sit up. He doesn't want to crawl on all fours either, he crawls like a wounded soldier. Of course, he is a little chubby, but relatives are making me worried that something might be wrong with him.