After looking closer at your list, it seems like the 'Robert' might have been his father, not a sibling. (I was taking a guess at the sibling relationship from the chicken scratchings on the back of an old photo - it appears that there is a relation - but it was not specifically given). Is there any evidence in the Gen. Soc. Mayflower Desc. that Robert T. was in the military - perhaps a Vice Admiral?