People today have NO IDEA how people behaved and Acted in 1960. Getting pregnant out of wedlock was considered highly shameful, and many girls were sent away. Having Sex with a black man in 1961 was a felony in 26 states. (If I remember my research correctly.) Getting PREGNANT by a black man was something which might have gotten them killed in many states and ostracized in many others. It was about the most scandalous thing a girl could do. That Madelyn Dunham's co-workers claim that she never mentioned she had a grandson makes it seem likely to me that Kansas girl Madelyn was very ashamed of it. Hawaii may have been very racially tolerant, but much of the nation wasn't, and I suspect Madelyn's parents were aghast because of their Upbringing.
All of this makes it completely reasonable to wonder whether Stanley got Shipped off to Uncle Ralph, or Aunt Eleanor, rather than that she remained in Hawaii.
Alternatively, it could be that Stanley never told her parents about the Marriage or that the father was black, it may be the result of their discovery of it after the child was born that CAUSED Stanley Ann to Leave quickly for Washington. That scenario has plausibility as well. The "Marriage" occurred on Maui, probably to keep it a secret. None of Barack's friends were aware Stanley was even involved with him.