I finally left a sippy cup of water in my toddler's bed. Water does not cause tooth decay, and if that sooths him then she should do it. I have worried about choking - but he doesn't choke or asperate in the day and it let me get some sleep. So I did it (probably around 14 mo). Look at all the people who put their kids to bed with a bottle of milk. I have yet to hear the stories about choking. As a toddler he is most likely able to cough out anything his mis-swallows.
The only "bad" thing about it is that he has a very wet diaper in the mornings (but Huggie's Overnights diapers hold it all in, most of the time).
I also look at the fact that my husband and I both sleep with water on our nightstands, and access it throughout the night. I also remember as a kid, having those half dreams of glasses of water because I am waking up thirsty. I finally decided a sippy cup of water is fine in the bed. We started about 14 mo old, and he's 2.3 months now and the habit it set.
Best of luck to your sister and you.
State Certified Early Childhood Special Education Teacher
New Mom as of March 2009!