I think the signatures have some factors in common.
Although not every corresponding letter is identical, there is a certain overall style which both have.
Some individual letters as I see it, are very much alike- the big bellied R and the stroke of the t which misses.
As a left handed person, myself, I wonder if the writer was left handed.
It was highly desirable in the well-educated script, that here be a forward slant to the writing. Left handers find this difficult and I do note the desired slant is absent in both signatures.
ADDING- Can you comment Betty?
As to the ages of the siblings and Robert's place in the time-line of them-
At this stage the evidence is showing he is a nephew, not a child of Walter Earle SEALE.
Birth of Robert Edward SEALE (proposed as
father to Robert who married WILKINS).
from India Births and Baptisms
Name-Robert Edward Seale
Christening on- 18 Jan 1843
At Nusseerabad, Bengal, India
Birth Date 09 Sep 1842
Father's Name- Robert Henry Seale
Mother's Name- Sarah Emma
This link posted earlier by CoL gives the same parents for Walter Earle SEALE
https://www.myheritage.com/names/walter_sealeWith that in mind, it would be easy to be confused or to make small fudges of the truth

Sue