Do you have access to the censuses?
Have you tried searching on Family Search?
As already shown Michael is the son of Robert.
Robert appears in 1841 321/24/43
Robert is 13 yrs with Mary 30yrs and other children, including Mary, Elizabeth and Ann. No relationships are given in 1841.
1851 2397/185/52
William and Mary Usher with children as above and Forster. Jamar has linked to this entry. FS haven’t shown the full family as they run over two pages but you can look the parents up now.
It would seem, therefore, that Foster is the younger brother of Robert and uncle of Michael. (I think)