Hi and congratulations on your new little one
I'm totally the last one to know the answer to this, lol because I'm having the same issue with my Sunni. I got her when she was about 9 - 10 weeks old, and yet she still acts the way you are describing. She's gotten much better but her possessive jealous streak still rears its ugly head once in a while. I do take her to a weekly training class which has helped get us to the point we are at today. I've been working on giving her treats when she stops growling at my other dog or if she sits nicely while I'm with him or my son for that matter. I dont always have a treat handy, so I do praise her in a very happy excited voice when I catch her being nice.
There are others here who will definitely be able to guide you. But its awesome that you saved a beautiful little furbaby!!