According to the pre-1855 MI Book for East Stirlingshire, there are 2 stones in the Churchyard to Sinclairs but not the ones you've mentioned.
Listed is:
1.
1851
William Sinclair and Agnes Christie (most likely his wife)
and next to the above
2.
W S A C (probably William Sinclair and Agnes Christie)
There are a further 26 stones similar to No. 2 (above) which only bear sets of initials (The surname could be anything beginning with S), but none of them have either I S or J S and an M ?.
(The letters I is quite often used instead of J).
This doesn't mean they aren't buried there, simply that there isn't a stone.
There are however, 2 Sinclair stones in Holy Rude Churchyard, they are:
1.
John Sinclair died August 1864
Mary Robertson his wife died February 1865
Son John died 9.11.1920 age 66
His wife Helen Miller died 19.6.1940 age 80
Thomas Robertson Sinclair, M.B., Ch.B., died 10.4.1929 age 40
John Sinclair died 3.3.1960
His wife Agnes Paxton died 1.3.1961
William Miller died 6.2.1896 age 57
2.
Duncan Sinclair died b. 9.7.1806, d. 15.2.1883
Flora Duncan Sinclair b. 16.9.1846, d. 17.2.1847
Janet Brown Sinclair b. 18.11.1818, d. 8.1.1876
John Sinclair b. 23.9.1849, d 27.1.1874
Janet Sinclair b. 25.6.1856, d. 26.6.1874
Ann Boyd Sinclair b. 10.08.1853, d. 10.3.1876
Charlotte Campbell b.4.9.1842, d. 25.12.1907
Annie Campbell Jack b. 3.5.1878, d. 3.5.1936
James Simpson Jack b. 31.12.1877, d. 14.2.1950
Erected by grandson John McIntosh Sinclair aged 71 in 1942
These MI's are all abbreviated so in the case of No. 2 (above) it's very difficult to figure out the relationships between the Sinclairs, Jacks and Campbells
Anne.
P.S. I also checked St. Ninian's New Churchyard (which surrounds the Church) but there's no Sinclair stones listed there either.