My Dwarf Hamster is Scaring the life out of its sister!?

i got two dwarf hammys ( female ) from petco a few days ago and they were in the same tank at the store. silver is chasing sunny all over... when sunny finally hides in a tight area behind their igloo, silver climbs on top of the igloo and jumps on top of sunny because shes too fat to reach in the tight place. sunny is scared of her sister. she is squeaking and silver is BITING sunny. i was all like, WHAT THE!!?!??! and now sunny is terrified! WHAT DO I DO!?!?! i don't wanna return silver :(