Apple ID auto log in
Same AppleID, eh?  Would you get each others' Facetime calls?  Would you be responsible for paying for her "in-game" purchases?  I guess you can set up "Parental Controls" (with you as the parent) to limit what she can do.  But is that really what you want?  Why not let her get her own AppleID?  There is a "family plan" for sharing downloads.