Any tips on sparring with a taller southpaw?

My brother and I train with a personal trainer(strength training and boxing), just for exercise but recently we decided to spar on Fridays. It’s very light but I love the workout.

My brother is a southpaw, probably 3 inches taller than I am, and 40 lbs heavier. I’ve been boxing on and off for a few years but my brother has not, so our coach is sort of cornering him while sparring. Now he’s teaching him to keep me away using his jab, and it’s really working. I find myself without solutions in the ring and tend to just put my head down and step in so I can land some shots, usually to the body.

I know I'm supposed to keep on the outside of his lead foot and I'm going to try and use my speed advantage to counter after slipping outside the jab.

Do you have any tips on how to spar against a taller southpaw?

Edit: Sorry, not 3 feet taller than I am. 3 inches. We use the metric system. 179 cm - 77 kg vs 188 cm - 100 kg.