Thanks for your feedback!
Yes, we're using ChatGPT 3.5 for now and we will certainly switch to GPT-4o mini in the near future. It already works with ChatGPT 4 but we noticed that the cost increase was not worth the minor improvement of quality.
I tried your example with ChatGPT 4 and it's somewhat better but not perfect: "I've even come to hate my brother and younger brother."
We hope that the quality will improve with the cheapest (and also fastest) models available.