B.C.
She's too young for all this stuff.
She's proven to be unreliable with her phone - she'd better be without it for quite awhile - and lock up the devices at night so she's not tempted to sneak them.
She's got too much time on her hands.
Get her busy and run her ragged.
Sports, dancing, arts n craft classes, swimming - keep her scheduled to the max.
Additional:
Sounds like your older daughter could use some grounding and losing phone privileges too.
Pimping your little sister isn't cool.