Next month I plan to make a trip to the area (I live in Canada) to look for more details of the siblings and their descendants, and ideally to find John and Betty Millar's parents (unfortunately I've not been able to find any of their complete BMD records that normally give that information, only a one-line entry for John's death).
Do not arrive at Inverquharity expecting to find much in the way of local records, and do as much homework as possible before you leave home.
All the statutory BMD records, most surviving pre-1855 church records and all the censuses are held in Edinburgh and can be accessed in the Scotland's People Centre there. See 'Visiting Scotland's People Centre' at
https://www.scotlandspeople.gov.uk/content/our-charges. Far better to spend a day or two there looking at the records before you head for Angus.
The best way to go about this would be to use Scotland's People online before you arrive, of course.
Once you have investigated all possible avenues at SP, you can then consider other options. There may, for instance, be something in an archive somewhere. See
https://www.nrscotland.gov.uk/research/catalogues-and-indexes and make sure you search all three catalogues thoroughly, because some archives require you to book in advance, and in several you have to order the items you want to look at anything up to three days before you get there. You may also need proof of identity - read those sections carefully so you don't turn up without the necessary admin bumf.
You are extremely unlikely to find anything more about the deaths of anyone who died before the start of statutory registration in 1855. However there is a possibility that your Millars (and don't forget to check for them as Mill
er too, because spelling in the 18th and 19th centuries was very variable) might have become involved in the affairs of the church, in which case there could be a mention of them in the Kirriemuir Kirk Session records, or just possibly the Cortachy KS records, because Inverquharity is very close to the boundary with Cortachy. These can be viewed in the Historical Search Room in General Register House, Edinburgh - see
https://www.nrscotland.gov.uk/research/visit-usAs your Millars didn't own Inverquharity Farm, it's unlikely that there is any reference to them in the Registers of Sasines, but it might be worth checking the indexes in the Historical Search Room, just in case.
Make sure you check Angus Archives
https://www.angusalive.scot/local-family-history/ in case there is anything there.
I checked the booklet of Angus pre-1855 gravestone inscriptions, and there are none in Kirriemuir that mention Miller or Millar in Inverquharity. Nor are there any in Cortachy, which is the neighbouring parish.
And this
https://www.geograph.org.uk/gridref/NO4057 might be of interest.
Oh, and under no circumstances plan to drive a car in Edinburgh. It's difficult to move, the one-way systems are bizarre, it's all but impossible to stop anywhere near any place you might like to visit, and if you are lucky enough to find a parking spot, it will cost you an arm and a leg. Use the city bus service, which is excellent, and don't hire a car until you are ready to head out for Angus.
If you have any more questions, feel free to ask.
