Adopting my younger sister- what to do?

I’m in the same situation. Mom abandoned me when I was Lil. Took me in age 11. Kicked out by 14 yrs. I went Awall. Then found my husband with whom I now have 3 children with. About to start our life. New house, starting a business. My mom is so unfit. She has epilepsy & she’s drinking to the point of falling over on a cement basement floor. & she just cheated on her 13 yr relationship & baby father of my little sister 3 yrs ago. to be with a druggy (Hard drugs) & my moms mom, whom my Lil sister has never met a day in her life; until she moved into our moms house renting btw only $850 monthly in Merivale. & she still can’t fill her fridge with food. Now she’s living with more druggies & my sister looks depressed every time I see her. Sister also just broke her leg at park unsupervised & now is at her dads. Since Friday until Sunday. At my moms, dads house. (Our grandpa) whom used to rent to two teenage crack heads, & my ex. step-dad was a drunk skunk before our mom kicked him out. I’ve been worried about her for months. Too much to explain but I’m thinking about adopting her in the near future. I have to look into it more because I’m not sure of any costs. I also have 3 kids of my own & I would love the help from my 10 yr old sister. I’m 24