I'd LOVE to share info with you. I don't want to post my email online, but I think it's available on here somewhere? (I'll check on that.)
At any rate, yes, William Mein (b 1800 in Wilton, Roxburghshire) and Eleanor Bell (b 1806 in Alnwick, Northumberland) were my ggg-grandparents through their daughter Mary Mein. Mary and two of her sisters (Ellen and Hannah) married doctors and emigrated to the US. A fourth sister (Jane) married a friend of one of those doctors in Washington state and also emigrated. Brother Andrew was a grocer and tea dealer in Liverpool, and William was a railway secretary or some such thing in Barry, Wales. Agnes remained unmarried and lived all her life with the parents. Finally, there was a brother named Robert Bell Mein born 1834 in Alnwick who disappeared off my radar completely.
I don't know which line your father-in-law descends from but I may have info about his ancestor. I do have mimeographed copies of about half a dozen letters written by William Jr. to his sister Mary in the late 1800s/early 1900s, and I've been working on researching the references made in those letters.
The Meins are one of my two favorite lines, perhaps in large part because I love the Borders area so much.
I look forward to collaborating with you and your father-in-law.