Sally has forced Sonic into becoming her Sister
as you can see Sonic isn't too keen on the idea
So to make sure he doesn't change his mind Sally has handcuffed them both so they can be to together always....

Sally: Isn't this nice Sonic, we can finally spent some quality time together
Sonic: yeah I grass so....( Help me... )
Sally: ( you do know as we are Sisters I made us able to read each other's thoughts )
Sonic: ( Crap! )

how did she change Sonic into a female chipmunk? why did she make Sonic her sister

all will be revealed in Sonic the Mad Doctor Acorn (as soon I finish writing script ) and yes (spoiler) its a was really a TG Story in disguise....

Enjoy