There were sizeable numbers of Neish known to have lived in this are since well before 1715. Yet more with bizarre medieval spellings. Because we cannot yet prove who is related to who I will ignore them for now. You may understand this better as you read on:-
Back further into history - So, we will start with Alexander Neish Born about 1715 died after 1778 married Ann or Janet Reid born about 1715 in Rothiemay Banffshire, Kids and our start for this family line were:-
Isobel 1737 - after 1777 Married John Loban 1766, 3 kids Janet 1768, Jean 1773, Margaret 1777.
Robert about 1739 - before 1801 Married Jean Taylor1769, Marnoch 4 kids, Alexander 1769, William 1772, John 1774, Janet 1778
Alexander 1741 - after 1769 no marraige or kids recorded
William about 1744 - Married Margaret Marshall born 1750 in 1776 , 7 kids Janet 1777 - 1858, John 1779 1861, Isobel 1782, Alexander 1786 - bef 1855, William 1789 - 1865, Elspeeth 1791 1858, Robert 1793 - 1870.
John about 1746 - 1791 Married Isabel Mcwilliam born bef 1748 and 3 kids Alexander 1774 - 1853, John 1778 1866, Robert 1781 - 1872 . There is also a child by Janet Reid - William 1769 in King Edward aberdeen. No other details.
Even by this stage the family is quite large all living close to the Rothiemay area of Banffshire. It is probable that the first Alexander Neish was a tenant farmer at a place called "Fowlfolds" in Rothiemay. (This was a Farm around a Duckpond, which still exists and neighbouring ruins listed by Aberdeen council as an ancient ruin still exist although the current owners of the area do not allow access.) The point here being the concentration of Neish with similar names in a very small area. I believe the references to Marnoch are where the records were kept. The same naming scheme continues throughout the extended forefathers and offspring who I haven't extrapolated out. They all seemed to live within a days walk of each other making it very difficult to sort out John from John especially when the records are centralised as happened in all the parishes.
So we now have your line, Alexander] then son Robert then son John 1774 marrying Margaret Fraser in 1801 who appears to die in childbirth or shortly thereafter about 1816-1820. Their kids were, Jean 1801, Alexander 1803 - 1870, John 1805, Mary 1811 - 1892, William 1815 and William Dason Neish in 1816. After Margaret's death we see Jane Horn but he (John) never marries her. John and Jane have 2 kids James 1822 - 1864 and Margaret 1822 1886. No ref to twins though.
So we now have the Mary 1811 1892 married at Rothiemay 21st October, 1832 to Alexander Henderson and having 7 kids. John, 1832, Alexander 1838, Janet 1841, Jean 1844, William 1847 to after 1892, George 1849, and Mary Grant henderson 1852.
The Margaret Neish you mentioned would have been Mary's half sister, married twice Bejamin Robb 1805 - 1842-3, then Francis Walker 1805 1885 and having 6 kids. so John Neish 1774 was her father. Her cause of death senile decay (better than the alternative name of brain fat)
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