Question

how are genes and proteins similar to homologus structure when determining evolutionary relationships among species?

Answer 1

Homologous traits of organisms are due to sharing a common ancestor, and such traits often have similar embryological origins and development. This is contrasted with analogous traits: similarities between organisms that were not present in the last common ancestor of the taxa being considered but rather evolved separately.

Answer 2

Both are extremely similar but have small changes that make all of the difference.
You've probably heard about how our bone structure is similar to many other animals', and likewise about our DNA.

When you observe these small changes, you will find organisms similar to each other where one little thing changes, and then another organism where there's another change, but that first change is also present. You can interpret evolution over different species just by looking at what's different in the bone structure as well as the genes and proteins.