In the last section of "Tintern Abbey," Wordsworth focuses on his sister, Dorothy. What does looking at Dorothy make him remember?

In this famous reflection on the delights of nature and
how nature is capable of being the balm to our exhausted beings, Wordsworth focuses on
his sister Dorothy, who is with him on his walk as he views the beauties of the
countryside before him. Note the answer to your question that the poet
gives:

My
dear, dear Friend; and in thy voice I catch

The language of
my former heart, and read

My former pleasures in the
shooting lights

Of thy wild eyes. Oh! yet a little
while

May I behold in thee what I was
once...

Thus we can see that
looking at his sister and her "wild eyes" makes him remember the way that he was before
in his passionate youth and how he regarded nature then, which is something that he had
already covered before earlier on in the poem. Regarding his sister thus helps him to
recall how he was once, which emphasises how much he has changed in the
interim.