I have been on Scotlands People today and found no reference to Marion Pennycook being a witness to any wedding, death etc.
On that note, who is usually a witness to a birth? Brothers, fathers or friends perhaps. Either way, it always seems to be a male. There were a couple of names unknown to me, so perhaps family friends?
It's unlucky Marion wasn't witness to her siblings' marriages but it doesn't mean she wasn't their full sibling.
In my experience, the father is usually the 'informant' of a birth, however, depending on the occ. of the father, who may be away from home then the mother or other relative.
In the case of illegitimate children, usually the mother is the 'informant' or another family member.
When trying to find DNA connections, it's best you build a 'wide' tree i.e. include all siblings of Marion, their wives/husbands & their descendants.
This was how I found connections to my maternal line.
Sometimes when you find DNA matches you may not recognise surnames in their trees who may be descendants of siblings of Marion i.e. tracing down those lines will help when you add them to your tree as quite often you'll find many haven't traced back far enough, hence the lack of the surname especially as mentioned, their ancestor was a female.
Annie