Sorry I have not got back to this- I have containers arriving!
Just a quick thought - have you tried checking for Isabella ( and variants) WAITE?
Someone else has the Mathew W marriage on their Tree ..also, maybe check to see which WAITE/WAIT/WATT families are around the area where the first HEPBURN children were born.
I presume you are searching Scotlands people? When it gets this tricky, it costs a bit!
Always check for variant spelling, and naming patterns, then work backwards, then forwards and sideways. Its often surprising what you find when searching family not directly connected to the one you want so go off on tangents!!....I noticed some WAITES went to the US quite early, but haven't pursued them
Keep me posted on any developments as I hope to re-visit this when the dust settles around here