Puppy is learning toy-possessive habits from our other dog and I want to stop both of them from acting this way.

I have a 10 month old Corgi puppy, and my sister (whom I live with) has a 6 year old Malamute mix. The malamute, Dexter, is very possessive over toys and even gets to the point of being somewhat aggressive if my puppy, Finn, attempts to grab the toy or play with it. Dexter will bark and snap and generally guard the toy when Finn even so much as goes near it (even if Finn has a different toy).

We've dealt with this in the past by simply removing the toy in question, because he does it with ones that are higher value to him (like treat dispensing toys and antlers). So we remove the toy he's possessive over, he generally stops. However I think the toy possessiveness is rubbing off on my puppy, because when we go to visit my parents, who have an 11 year old golden retriever, Finn will act possessive and snap at the golden when certain toys are involved. If the golden is playing with a rope, for example, Finn will go up and grab the rope and they'll play tug. However if Finn gets possession of the rope, he will refuse to let the golden have it back and will bare his teeth and get kind of nasty if he tries. I'm familiar with Finn's play-growl, and when he growls at the golden for trying to take his toy, it's definitely not his play-growl.

Finn's already learned some really bad habits from my sister's dog (barking to be let in non-stop, getting really excited when people get home- something I had been trying to avoid-, barking when people come to the door). They were all things I had started to train him NOT to do, much like making sure he was good about sharing toys, but since Dexter is so bad about these things, Finn is picking it up.

I could use some tips on maybe getting BOTH of them to stop being toy aggressive, but more importantly I want to figure out a way to make sure Finn knows that it is not okay to snap at another dog for going near his toy. And just as another aside, he is absolutely not toy aggressive toward me, he doesn't care if I take the toy away, so taking it away and treating him for letting me take it probably isn't going to work.

TL;DR: My sister's dog Dexter and my puppy Finn live together, and Finn has picked up some toy-possessiveness from Dexter and is now aggressive toward my parents' dog if he tries to play with a toy Finn has. I want to stop both of them from being toy aggressive.