I've also had friends -- and once, a boss -- who has bi-polar disorder. I think this is very good, especially the way the last line breaks the rhymes and ends the "episode."

I guess if I had a suggestion, it might be to play with the rhyme scheme.
It's aab all four times through, but on the last two verses the bs also rhyme, which for me slowed the manic feeling of the fast, hard rhymes.


It might work to have the rhymes come faster -- and harder -- the further you get into the poem. Maybe a verse of abcd, then abca, then abaa, then aaaa. Then the verses stay all "a" rhymes but get a line shorter each time. When you get to the last verse it's just one line but all the words rhyme -- hard, short rhymes.
Then the driving back home to break it.

That kind of rhyming would be a challenge for sure, but it would build the idea of becoming increasingly manic.
If she's driving down the road, there could be lots of images to play with.
Maybe the images could change more quickly as you went along, as well.

Anyhoo, just a thought. I also think you have a lucky sibling.