Hi girl,
I talked to my friend about your dillemma with Cassie. She is an
amazing Kindergarten teacher and department head, and here's what she
said...

You cannot, at Erin's age, really teach her to overcome Cassie's
manipulation, but there is a solution. Instead of letting the girls
play freely, you can orchestrate their time together more. Plan a
craft, a story, a movie, a snack, a kitchen activity, or a board game.
Keeping them busy and staying involved yourself will lend enough
structure to their time together so that BRIEF times of free play will
be less likely to erupt into a power struggle. If Cassie tries to
manipulate you by saying she doesn't want to participate in what you
have planned, she can lie down while you and Erin have all the fun.

Of course, this means less free time for you, but that's the way it
often goes with little kids.
Hope this helps.