I can accept that - especially if it gives her name so we can definitely assume that Marianne and Eliza are sisters.
So... going back to those baptisms of Eliza's children and Isabella Deedes Pyne.
Emma, Charles and Frederick (Eliza) and Isabella (Mary Ann) were all baptised on the same day with no fathers given.
This could mean that they were illegitimate or some other reason (absent in military or similar) but it is odd.
It's all a big coincidence that two Pyne sisters would marry two Pyne men - but again it is possible.
Emma and Virginia are traceable in the censuses and both marry well as do Marianne's children so again things are contradictory. It doesn't seem possible that the children are all illegitimate with whatever connotations that would bring.

I have just seen 1861- Augustus F Pyne b 1826 London a Lieutenant in the Navy - I wonder if he could be Frederick?
I also saw earlier an account in an Irish paper of 1850 there was a Charles Pyne -Navy being court marshalled in Portsmouth - I think or Plymouth but nothing else.
Justin's ancestor is Virginia- I wonder if her marriage certificate has her father mentioned?
Well that's about it for now- a real puzzle but so interesting.
Goodnight
Kath