Just adding to this, belatedly, to say hello!
I descend from Samuel Sibly 1734 St Cleer -- him who was the parish clerk, I believe, and "a worthy man", as his burial entry tells us. BMDs in the blood! (And I'm sure Sam would be rolling over in his grave if he saw the dozens of trees at Anc'y and elsewhere that have him fathering a son in Tennessee in 1775. Those Anc'y hints ... aren't they great.)
Samuel 1734 was the son of Thomas Sibly and Mary. This would make them Thomas Sibly and Mary Sibly (so it says) who married 1731, I presume. I have not done a lot of poking around in this line, as it is well researched by a lot of people. Also, the online parish clerk for St Cleer is terrific, and I've written to tell him so! Oh, hold on, it has a new one -- but I'll bet she's terrific too.
The links here
http://www.cornwall-opc.org/Par_new/parishes.php/are not working for me, so I can't tell what's there now.
I believe my Thomas Sibly is the son of Samuell and Mary, born 1713, rather than of your Thomas Sybly son of Richard and Grace, born 1703.
So my Samuell would be born 1672, son of Thomas.
If your Richard was born 1668, son of Thomas (plus there was eldest child Thomas 1665), I'm betting they were brothers.
Hard to tell, when an entire clan over a couple of centuries only has three names among them ... !
But one way or another -- howdy, cuz! from this branch of the family in Canada.