Consider you have a Parent Frame(Frame-Parent). and 2 child frames(Frame_Son,Frame_Daughter). Lets see various conditions and how to handle.
1.From Parent to Son or Daughter:
driver.switchTo().frame("Frame_Son");
driver.switchTo().frame("Frame_Daughter");
2.From Son to Parent: If parent is default frame, switch to default frame, else from default frame switch to parent frame. But you cannot switch directly from son to Parent.
driver.switchTo().defaultContent();
driver.switchTo().frame("Frame_Parent");
3.From Son to Daughter: If your sister does some mistake don't yell at her, just reach out to your Parent. Similarly, you give control to parent frame and then to daughter frame.
driver.switchTo().defaultContent();
driver.switchTo().frame("Frame_Parent");
driver.switchTo().frame("Frame_Daughter");