Well, having a creature enter is decidedly easy in white, especially when you have plenty of creatures that can summon forth tokens or act like Restoration Angel anyways
With Mockingbird you get the upside of a flying clone, and its wording means you're always spending a net-even amount of mana compared to the creature you copy
Subscribe for unlimited scans and valuations
The problem with Sliver Queen isn't so much the power but the accessibility to her
Standard has access to quite a few pieces for a Soul Sisters deck such as Lunarch Veteran, Cleric Class, Trelasarra, Moon Dancer and Cleric of Life's Bond
You can try to bash through with Ivory Giants, but many of your best creatures are nonwhite so it can be hard to use aggressively