"tender" means "young" in the poetic sense. "sequoia" is a type of large redwood tree that grows in California, I think. So, the poetic piece, "as you move through time, layer by layer, tender sequoia" is poetically comparing the young cousin to a young redwood tree that gains layers and strength as it grows through the passage of time.

A partial answer: https://forum.wordreference.com/threads/the-meaning-of-tender-sequoia.1234769/ A Sequioa is a redwood tree that grows to be hundreds of years old in the northern mountains of California (Yosemite). "Tender" in its literary sense means something that is young.