Hi GMac,
I apologise to have taken so long to reply.
I've checked my info here over the past few days, for Roderick Chisholm & Christy McGregor. I believe Margaret is definitely a daughter with brothers Duncan, John & Donald. But even though Catherine is shown with the same parents on the burial record, I haven't been able to find any other proof she is a daughter as well. It seems likely, and I didn't find anything to disprove it, but they lived quite a way from the other family members.
I couldn't find Hectors death, but it was between 1881 and 1891 censuses, most likely at Opinan as well.
You seem to have the marriage record saying Catherine was from Ormiscaig, which makes it possible Catherine is a sister as well.
I had hoped to be of more help, and wish you well with your research. PLease don't hesitate to contact me again if you think I can be of more help.
Regards,
Ann