Yes, I did receive several responses. It turns out "Sinker Creek" is not the correct name for the location of the Utter / Otter Massacre. The Indian attack took place at Castle Butte (east of Sinker Creek), on the south side of the Snake River, about 50 miles from my parents' farm.
Donald H. Shannon in 1993 published a book, "The Utter Disaster on the Oregon Trail," which thoroughly explains the above event. The book includes maps showing exact locations. (The "Sinker Creek" location was named in an earlier book on the subject. Apparently my mother read that book, giving her incorrect information.)
Thanks for your response.
Mary Foster Ludvigsen