Hi Impy and Gadget
Bear with me whilst I set down my theory
Pure spec here, but I think the reason you haven't been able to find out much about the family may be down to their surname changing. With the details for another possible sister Margaret born in Scotland c.1834 showing in the English 1871 census, I searched the English 1841 census for her by first name only.....and up popped I think the family under a slightly different surname. Everyone showing as born in Scotland living in Leath/Penrith, Cumberland:
Samuel Gallahar 50 (can't easily make out his occupations)
Mary Gallahar 40
Peter Gallahar 15
Joseph Gallahar 15
Catherine Gallahar 13
Margaret Gallahar 11
Agnes Gallahar 8
Now searching the OPRs for possible births, brings up this - so happens all in Dumfriesshire
I've highlighted all the possible names that you have shown so far as siblings for John, as you can see they also show:
1 08/11/1815 GALLACHAR
JOHN SAMUEL GALLACHAR/MARY WILKINSON FR320 M Graitney or Gretna /DUMFRIES 827/ 0010 0334
2 27/02/1818 GALLACHER JEAN SAMUELL GALLACHER/MARY WILKISON F Ruthwell /DUMFRIES 846/ 0010 0182
3 06/04/1820 GALLACHER
EDWARD SAMUEL GALLACHER/MARY WILKINSON M Ruthwell /DUMFRIES 846/ 0010 0185 (second entry with slightly different spellings: 06/04/1820 GALLHAR
EDWARD SAMUELL GALLHAR/MARY WILKISON FR196 M Ruthwell /DUMFRIES 846/ 0010 0185)
4 22/06/1822 GALLACHER PETER WILKINSON SAMUEL GALLACHER/MARY WILKINSON U Ruthwell /DUMFRIES 846/ 0020 0009
5 20/11/1826 GALLAHAR CATHERANE LETIT SAMUEL GALLAHAR/MARY WILKISON F Dalton /DUMFRIES 818/ 0030 0009
6 06/04/1829 GALAHAR
MARGARET SAMUEL GALAHAR/MARY WILKISON Dalton /DUMFRIES 818/ 0030 0012
7 10/07/1832 GALLAHAR
AGNES SCOTT SAMUEL GALLAHAR/MARY WILKISON F Dalton /DUMFRIES 818/ 0030 0019
8 08/02/1835 GALAHAR WILLIAM SAMUEL GALAHAR/MARY WILKINSON M Dalton /DUMFRIES 818/ 0030 0023
Regards.
Monica