Check the law in your state. In MD they’re too young. They have to be 8 to be left alone and 13 to be responsible for another child.
Even if you’re not in MD it’s a good guide, then use your kids as a guide, time of day, maturity level, are they anxious, and SN etc….
I wouldn’t leave a 10yo responsible for a 6yo for more than about 5 mins.
I have an only and she is sometimes alone for upto 2 hours, but not very often.