Isaac and Daniel were most likely brothers as they are mentioned in a Union Co., SC deed dated 1793 as the "rightful heirs to the real estate of Neal McKissick". I believe Neal was their father, but have no proof yet. I have a fair amount of info on the McKissicks- although still consider myself in a McKissick muddle.
It appears as if Mary was indeed a daughter of Isaac McKissick- the following is from Isaac's Union Co., SC will dated 27 Jan 1832, and proven 10 July 1834:
"...I give my silver watch to wife to be taken care of until my grandson, B. F. Foster is capable of taking care... my son-in-law Josiah Foster to advise and assist my dear wifein the due execution of this my last will and testimony."