The only written reference to Mabel Elizabeth Robinson is in a letter from Bill Robinson of Texas in 1988 (Will's son). Anna Lynch did not have any recollection of her father (Charles) having a younger sister who died just after her fifth birthday, even though there is a gravestone in the Pine Grove Cemetery in Marlborough with Mabel's name on it.
The following is from Bill Robinson's letter:
"As you probably know, my dad had a little sister that was born when Grandma Robinson was fifty years old. She was accidentially scalled to death when she was twelve years old." (the entire letter can be read on William Orrin Robinson Jr's biography page)