I went through a very similar time when DD2 was new.. I dont think they have to see it anywhere its just insinctual when they cant verbalise thier frustration at the change in thier world. The ony time i remeber actualy taking a toy away was when she threw a toy in anger and it went through the lounge window.

There were a couple of things i remeber helping, making a show of telling DD2 "lets sit and play with DD1 now shall we" or "Not now DD2, I need to do XXX for DD1 first" I tried to choose a time when DD2was nice and settled and did not need anything so it was for show rather than anything but it told DD1 she was coming first for a while.
I also used to ask her to choose DD2s nappy or clothes to give her some control too, as many plat dates as i could ith her friends.