Also, there is a family tree on a/try that you could check (you need a subs to view it
www.ancestry.co.uk/family-tree/person/tree/28350218/person/12037695199/facts?ssrc=They show another potential sister called Jane/Jean (common first name variants) born c. 1794 in Glencairn. Jane is listed as having married twice, a Joseph Milligan and then an Alan Burgess.
Her death is showing as being on 19 Dec 1878 in Balmaclellan, Kirkcudbrightshire.
No details on her parents included unfortunately
Not sure how you can verify that these two women, Margaret and Jane, are sisters to your William. Sometimes occupation on a death cert for a father can help. If it shows as agricultural labourer...not a lot of comfort there really

Monica