J.P.
The sippy cup is not a replacement for the bottle little one's do not need to go to bed with a drink and especially anything but water anyway if you choose to do that. Sippys are a transition from bottles to cups and should only be used at the times you would use a cup ie. at the table, getting a drink between meals. Kids will not let them selves starve and they will drink when they need one. The only reason they would "need" one to go to bed is if you put them to bed with their bottle and so it is more of a soothing habit they have developed. But a horrible habit for their teeth more than anything else. Any normally developed child can drink from a regular cup anywhere from thirteen month's to fifteen month's at the longest.
Good Luck,
J. P.