Is the cooling of the relationship between two brothers considered a severing of kinship ties (qati'at rahim) or abandonment (hajr), as long as they sometimes speak and cooperate?

A brother is a Mahram relative, and maintaining ties with him is obligatory. What you mentioned about helping your brother and the absence of problems between you two is a good thing, as it negates the forbidden cutting off of ties and abandonment. However, the relationship between you two should not be lukewarm, where you only speak to him occasionally. Rather, each of you should be open with the other, even if it requires effort, until it becomes second nature.