If I can get my sister to voluntarily give me guardianship of my nieces would we have to go to court or could a lawyer take care of it?

My 2 nieces have lived w/ me since birth (4 & 6). My mom & I have been sole caregivers & providers even though my sister also lives with us. We are on the verge of asking her to move out because of behavioral/emotional issues & she would not be capable of taking care of her daughters. We will give her a choice of staying & working through her problems w/ counseling & becoming a good mom to her daughters OR she can choose to live however she wants & move out WITHOUT her daughters. If she chooses to move, she will have to give me guardianship. She will not be happy but she has frequently expressed that she feels the girls have ruined her life, etc. but feels obligated. She is neglectful at best & has only contributed with some clothing & toys. At one point CPS was involved & she was not allowed to be alone with them (for about 3 months after she got out of a state mental facility - about 4 yrs ago). At this time we do not believe it is safe to leave her alone with them so we don't.