Forget about Ancestry. It's not much use for basic research in Scotland because it doesn't have any of the original birth, death or marriage records. It does have transcriptions (but not originals) of some of the census, but beware of some of the imaginatively creative spellings in the transcriptions, and make sure you always check them against the originals, to make sure that any transcription errors in your own tree are your own.
Duplication of names in a family is quite common. If a child died, the next one of the same sex was often given the name of a dead elder sibling, so there's nothing remarkable in there being two Marthas or two Georges.
According to the IGI at
www.familysearch.org George Hen(d)ry and Isabella Carstairs had six children: Martha Smith, 1855; Catherine, 1857; Jane 1859; Agnes 1862; George, 1866; Isabella Lamond, 1868, all born in the parish of Alyth, Perthshire. If they were married in 1850, there could well be other children not listed in the IGI.
You need to go to
www.scotlandspeople.gov.uk and invest in 30 credits at modest cost. Use one to locate and five to download the birth certificate of Martha Smith Hendry in 1855. This will tell you the ages and birthplaces of her parents, how many children her mother had had before 1855, and how many of them had died. You are in luck because 1855 is the only year when they collected all this information.
Deaths of married women in Scotland are indexed under both their maiden and their married surnames, and a bit of fiddling with the index at Scotland's People tells me that Isa* Hen*ry or Carstairs died in Angus in 1893, aged 67. Use another one credit to find and 5 to download the death certificate. This will tell you the full names of both her parents, and whether or not her husband was still alive. Use this to narrow down the search for him, and unless you are unlucky his death certificate will cost you just 6 more credits.
You'll still have twelve left which you could use to look up the family in a census, or to look for the death certificates of two of the four grandparents, if they have reasonably unusual names.
Noting Isabella's age at death suggests that she was born in 1825/6, and as she was married and died in Angus, and Alyth is just over the county boundary, she was probably born somewhere in or near those counties.
I note from
http://www.freecen.org.uk/cgi/search.pl that an Isabella Carstairs, aged 15, is listed at Sunnybraes, Dunino, Fife in the 1841 census in a household headed by John Carstairs, 45, farmer and Grace Carstairs, 40 with five siblings. I also note from the IGI the baptism of Isabella, daughter of John Carstairs and Grace Duff, in 1823 at Largo, Fife. This is not to say that this is your Isabella, of course, especially given the absence of a John and a Grace from that list of Hen(d)ry children, but she is the only Isabella Carstairs of the right age in the FreeCEN transcription.
Happy hunting!