I mean she has a inferiority complex when it comes to taking care of the kids so she probably thinks that Yuko will just take over and do a better job of it. I hope they can all learn and communicate like a real family.
Trying to replace your own motherless grief by taking care of an abundance of children at a young age is noble but has to be extremely stressful. That coupled with her inferiority complex and what she now thinks is her dirty mind. But running away won't solve anything and isn't fair to the kids or Shigeji. Then again, she is only 20. It's not like she's sooooo far removed from being a kid herself. She's only thinking of herself right now.