Taking care of two in diapers?

My mom checked herself into an In patient clinic for rehabilitation and detox of oxycodone and methamphetamines. She said she'd go in, as long as she could get someone to watch my 17 month old sister. My grandmother and I agreed that we would take turns (every other day, for 13 days)... but I'm still unsure, I dont know how I'd manage, but my mother can't keep going down the road she's on- foster care is not an option- do I tell my grandma that I cant?

: http://alcoholdrugstreatment.info/taking-care-of-two-in-diapers