Definitely nothing to worry about now. Instead of trying to actively teach them to sit, focus on freedom of movement. The more space they have to wriggle around and crawl and get in and out of different positions, the more they develop overall muscle tone and control as well as balance and proprioception. All of these are the building blocks for gross motor skills.
My daughter is almost 8 months and she just started to sit on her own a few days ago!!
Dont worry about it. Mine did not sit either. He started sitting around 8 months and was standing by 8.5 months. Every baby is different. I would not think too much about it, but I you are worried check with your family doctor 😊