Well... the way I see it is a fairly normal situation. I don't think it confuses Suzi at all. Everyone usually has "family dogs." Growing up, I got Daisy, our Golden Retriever, for my 5th birthday. Well, of course I was too young to take care of her myself so even though technically she was *my* dog... she was really my parents dog. She would love on me sometimes and come cuddle with me and give me kisses... but she was my dad's dog. She slept with him, she listened to him, he fed her, etc. So, that part of it sounds like a very normal situation and I don't think you should be jealous when Suzi wants to be with your son. You also have to think.. IF you get a dog for him, you will have to make sure he is the sole caretaker of her (or at least most of the time) so that the new dog would connect with him. If you're the one it, taking care of it, etc... it's going to look at you as it's owner as well, just like Suzi does.
Second, I think it'd be really cool if you got another Yorkie and don't see the problem!
But, I love having more than one dog so maybe I'm biased.