Custody in Wisconsin

Long read, just want some advice My sister put out her 16y/o & 5y/o daughters and now they’re at my house. Thursday evenin, my 16y/o niece contacted me from my mom’s phone asking if she could come stay with me. Not knowing the whole situation I said yes because in no way shape or form for any reason other than harm being caused by the child to the parent- should the parent kick out their kid. So once they get here I asked for my nieces side of the story and I immediately take her side because I’m well aware of how my sister lives and how she move when she got these new love partners. All of this to say that my sister kicked her out over my niece not washing the dishes that her and her love interest messed up. And wholeheartedly she’s not wrong for standing up for herself and setting boundaries. I know my niece cleans behind herself because she’s been having to clean and care for her two younger siblings for the last 8 -6 years of her life because her mother - MY SISTER - leaves them on her. Forcing her to watch, feed, and bathe them as if them her children while my niece misses out on being a kid. So today, I called my sister asking if she would come get them. She told me she would come when she got off of work at 6. I got a missed call from her @7:05pm but we were all taking a nap so I didn’t answer. When I woke up 30 minutes later and returned the call, She told me that because I didn’t answer she wasn’t going to come pick them up due to the fact she has to work at 10pm. It’s only 7:39pm at this point in our conversation so I asked again. “ you not coming to pick them up “ and she laughs and says no. I’m a big advocate of mental health and frankly, I don’t want to send them back home because it’ll be like my sister has won and gets to keep inflicting emotional trauma and damage onto my nieces. I don’t want that for them, I’d much rather step up and take the challenge so that my sister could learn that what she’s doin isn’t right. I asked my niece what she wants to do and all she’s concerned about is her clothes and school ( rightfully so ) but I genuinely don’t know how to move forward. I kind of want to wait it out and see if she’s goin to make the effort to retrieve them. The abandonment laws in WI, is 3 -6months of no communication. And knowing my sister, now that I have two of her three children she would be content with just knowing where they at and not speaking to them…. Please guide me in the right direction, for I already have two children of my own but I believe I can help them