The Cosman connection is just as clear as mud. Her will defines "nephews" or children of nephews deceased. If taken literally that would have to mean they were children and grandchildren of one or both of her sisters, Susan Jennings or Sarah Jane Spier. Susan is given in the 1880 census with a daughter Martha J. Simms age 11, called step daughter of Archibald Jennings.
To further confuse things Clarissa's brother William C. Rodman is enumerated with his cousin William C. Cosman in the 1875 state census. Worse yet there is another William C. Cosman floating around that area.
At this time I am thinking there may be another sister in here who somehow escaped being enumerated by name 1850 and later. Date wise there is 8 years between Clarissa 1828 and W.C. 1836 and no definite marriage date for their parents - there is plenty of room for another sibling on both sides of Clarissa.
I no longer think that her mother Martha's maiden name was Cosman. It might have been Caverly though.