When the chips are down means at the final or critical moment, but in this context the speaker is saying that their little brother is always there for them even when things are going smoothly
Context: when things are going smoothly and there are no difficulties, I have always been able to count on my little brother, that's a fact.
Hypothesis: when the chips are down, I have always been able to count on my little brother, that's a fact.

Given this context, determine if the hypothesis is true (Yes, no, or unknown)?