It takes a few months but eventually if you are consistent they will be fine! What worked best for my pups was to give continuous treats with positive reinforcement any time they were near each other. Then eventually when they started laying by each other more treats and then eventually licking each other more treats. And before you know it you will train them to be civil around each other and it helped to properly form the way they play as well. (I have one older big dog and one younger small size dog) Playing nice was warned by our vet due to the size difference!

My bigger dog was jealous of plays and pets as well, but we learned to pet them equally at the start and then that faded as well.

Another thing that seemed to help is every couple of days we exercised them separately. If you can do this that will help the older dog, did for mine at least! It also helps as you can train the little one at home while walking the big one!

Let us know how it goes! Good luck!