I think I'm begining to see where this family trace may have gone off line...I think!
1. From the info you posted Jean regarding the line up from your grandfather James, you said his parents were Robert Lowe and Ann Hamilton. Now, from SP, I believe Robert and Ann were married in 1882 in Bridgeton, Glasgow City (the only marriage showing so must be them).
2. Again, you say that Robert Lowe's parents were James Lowe and Elizabeth McNicol. I assume this is from Robert's marriage cert. This is the point that I think you may have gone wrong. It is here at least that we do have TWO couples with those names, one in Angus and one in Glasgow City.
3. For the Angus couple, there is a marriage entry on IGI and children showing:
JAMES LOW Marriages: Spouse: BETSY MCNICOL
Marriage: 01 OCT 1849 Saint Vigeans, Angus, ScotlandScotland, British Isles
Children:
1. BETTY MCNICOL LOW Christening: 23 AUG 1847 Kirriemuir, Angus, Scotland
2. ALEXANDER LOW Christening: 05 JUL 1849 Kirriemuir, Angus, Scotland
3. JESSIE WATSON LOW Birth: 25 JAN 1850 Christening: 06 FEB 1850 Saint Vigeans, Angus, Scotland
4. DAVID FERRIER LOW Birth: 18 DEC 1851 Christening: 01 JAN 1852 Saint Vigeans, Angus, Scotland
5. MATILDA LOW Birth: 14 JAN 1852 Christening: 02 FEB 1852 Kirriemuir, Angus, Scotland
6. JANE ADAM LOW Birth: 03 DEC 1853 Christening: 15 FEB 1854 Kirriemuir, Angus, Scotland
7. JAMES LOW Birth: 09 MAY 1857 Kirriemuir, Angus, Scotland
8. ELIZABETH LOW Birth: 02 MAY 1859 Kirriemuir, Angus, Scotland
9. MARY LOW Birth: 23 SEP 1861 Kirriemuir, Angus, Scotland
Now I have no idea if the James Low here is the son of William Low and Jane Peter (parents Andrew and Ann Nicol) without viewing death certs. What I have seen, from the free searches on SP, is a death entry for a Betsy Low, other name McNicoll, in Kirriemuir Angus in 1875 with a birth year of 1820.
4. For the Glasgow couple with the same names, I can't see a marriage entry on IGI, however, these are the children showing on IGI:
1. JOHN COUTTS LOW Birth: 27 APR 1856 Bridgeton, Glasgow, Lanark, Scotland
2. JAMES LOWE Birth: 11 MAR 1858 Bridgeton, Glasgow, Lanark, Scotland
3. ROBERT ADAMSON LOW Birth: 06 JUL 1860 Bridgeton, Glasgow, Lanark, Scotland
4. JOHN ANDERSON LOW Birth: 21 JAN 1863 Bridgeton, Glasgow, Lanark, Scotland
5. THOMAS LOWE Birth: 06 MAR 1865 Anderston, Glasgow, Lanark, Scotland
Now, I am thinking that Robert Adamson Low, No.4 above, is your g grandfather. It would also fit the time and place for his marriage to Ann Hamilton in 1882.
There is also a death of a Betsy McNicol in 1865 in Bridgeton, Glasgow City (on the year that son Thomas was born if we have the same Betsy). Her age is unrecorded so it was probably illegible on the cert and has not been indexed.
5. My next question is how did you make the connection from Robert Low in Glasgow to the James Low (father) in Angus? If you have gone only from the data on IGI, this may explain where you may have gone astray. There isn't any marriage showing on IGI for the Glasgow James and Betsy McNicol (only the Angus couple) and that may have added to your confusion.
If Robert's mother Betsy did die after/around the time she gave birth to last son Thomas in 1865, you could perhaps see that cert and check whether husband James is still alive or deceased. It would also say on Robert's marriage cert. whether his parents were alive or had already passed away.
There are two deaths, for example, of a James Low* (some spellings of the name are coming up as Lowe so probably good to use a wildcard to pick up on both versions) in Bridgeton with a birth year of 1825 +/- 10 years.
You haven't said, but have you looked at any census info on the family of Robert in Glasgow. This would also help confirm birth places for parents, in particular James.
I do think the issue of Jane Peter's parents (if indeed she turns out to be connected to your line) in not relevant at this stage. You need to go back and verify 100% the parents of James Low who was having children in Glasgow NOT Angus.
Regards.
Monica