「身長が」という意味です。

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: いちばん背が高い 背がいちばん高い And consider, 1　弟がいちばん背が高いです。 背が高い 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: いちばん背が高い 背がいちばん高い 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.

こんにちは。 背が高い　together means “tall, big (height)”. Other examples: 背がひくい　short, small (height) => 先生　が　いちばん　背がひくい　です。 the teacher is the smallest. あたまがいい　clever, smart => わたしが　いちばん　あたまがいい　です。　 I’m the smartest. Hope it helps🤗

🤗