********* Ancestry again! Never trust anything you find online unless it is a scan of an original document.
There are two baptisms of children to George Rennie and Janet Stroner in Dundee, Ann in 1840 and James in 1842. This couple are in Dundee in the 1841 census: George, 25; Jannet, 20; Ann, 1; all born in Angus.
In 1841 yours are listed as Rannie, and they are in Alva: George, 25; Janet, 25; James, 6; Mary, 4; George, 3; Esabella, 1; and Jean Stevenson, 20.
They are at Burnside, Alva in 1851: George, 25; Janet, 37; James, 15; Mary, 14; Isabella, 10; Janet, 8; William, 6; Marion, 4, all except Janet sr born in Alva. Isabella must have been born in 1840/1841 and Janet in 1842/1843, which is almost exactly when Janet Stroner's children Ann and James were born.
So it is plainly obvious from the records that they are two different families.
In 1861 they are in St Ninians: George, 45, Janet, 45, Mary, 22, Janet, 16; Wallace, 15; George, 6. (I think Wallace must be a misreading of William; when you look at the original you can decide for yourself what you think it is.)
I can't find them in 1871.
I bet someone has been looking for Janet Stevenson, come across Janet Stroner, and, because they can't find baptisms with mother Janet Stevenson, have assumed that Stroner is a variant of Stevenson and they are the same people. Then other people have found and copied that wrong information, and before you know it there are 20 trees online with the same wrong information.
If you haven't already done so, get a copy of the birth certificate of George Rennie, born in 1855. This will contain a load of extra information over and above what you would get in a later birth certificate. It will confirm that his mother was Janet Stevenson, and it will tell you how many older siblings George had, and how many were still living and how many were dead.
I see that George Sr, widowed, is in the household of George Jr and his wife in the 1881 census. Janet Stevenson or Rennie, aged 67, died in St Ninians in 1878. George Rennie, aged 71, died in St Ninians in 1887. If these are your George and Janet, their death certificates should tell you the full names of their parents including their mothers' maiden names.