Teaching your little brother to ride a bike is a crash course in patience. Mostly his crashes, some of yours.

Teaching somebody exposes your own limits faster than doing the thing ever did. The frustration that comes up is almost never about their pace; it is about the gap between how fast you expected this to go and how fast a six year old actually learns to balance. They are not behind. Your estimate was optimistic.

Watch for the moment you get annoyed and check who it is for. If the answer is you, say nothing, let them try it again, and keep your hand on the seat. Their timeline is the only one that matters here.