I am still don't know why after いちばん there is 背が before 高い。
いちばん in your 1 and 2 is an adverb. The position of an adverb is relatively free:
You say 高い. What is so? 背 (height).
背―高い, 背が高い, This is the most important topic in it.
いちばん is a modifier working with the topic "背が高い".
いちばん can be set:
But 背が高いいちばん isn't right.
Plus, in this topic, you're talking about your brother.
Your 2 is more simple. 2月―寒い. How? いちばん.
You can say いちばん2月が寒いです。also.