I just bought a 1x10" Marshall combo guitar amp, and I've read in articles that many guitarists sit on their amps.
Is it okay to sit on it? Is there any chance of breaking it as a result?
No, there is absolutely no risk. You may have seen such amplifiers being stacked on stage during festivals/concerts, and those burdens can get heavier than what you weigh. Just make sure you're not hurting your ears by being too close to your amp when it's turned up to eleven.
Depends on how heavy you are and where and how you sit. Amps are designed to be stacked corner-to-corner, and a cheap combo amp might be weak in the middle of the case. That said, I used to sit on mine almost every time I played. I'm only 120 pounds though...
I wouldn't think you'll break your amp - but you can get loads of hum with your guitar that close to the amp, especially if you're using overdrive. And, it's unlikely to be very comfortable! If you're going to be using it in a place where there is a chair available, I would use that instead.
Normally you can sit on your amp, but if I were you, I wouldn't do it, ill just sit in front of the amp, to have the best sound return.
I would generally say that there is no risk to your amp by sitting on it. This case could be a little different since you are using a relatively small amp but I still doubt that you would be able to break it unless you are significantly heavier than the average person. I also imagine that you would be pretty close to the ground, which I would find uncomfortable.