So here we have a case of "it's mine and mine alone", followed by "I will share with others".
I think that there is actually going to be more to it than that - I feel that what may be in dispute here is legal, more than moral, ownership.
It could be that, if Franquin came up with Gaston
in the line of his employment at the magazine, the magazine could claim that they, not he, were the owners of the characters and settings.
If Franquin, acting as an editor working for Dupuis, was either asked or decided that the magazine would benefit from commissioning filler material to be used to plug gaps in the magazine, then it might be deemed what is called in America "work for hire", where the rights remain with the company.
This is to protect businesses, which, having invested time and money in producing and promoting a work, are then left bereft if an employee walks off to a competitor taking the work with them.
It's probably quite a strong case, especially if what the Wikipedia article you link to is true, and that other known individuals contributed to the design and creation of the character. It's probably even stronger if the character can actually be shown to have been established as "working" at the magazine as part of his raison d'être
This is all not to say that perhaps Franquin's daughter doesn't have some evidence to back up her position - perhaps Franquin had some contractual claim over his work, and had it in writing that his creations, while working for Dupuis, remained his. He may have been big enough, and valuable enough, to have been granted special conditions, but only time will tell on that.
The fact that there is his period drawing Spirou
to be considered may also be used to back up assertions that he was working "for hire" for Dupuis, or perhaps that will explain why he later insisted on having characters he created deemed to be his - we must wait and see!
Marsupilami, having had an independent life outside of his source comic, and a seperate set of adventures in adaptations, may be yet again a separate entity, so that too could add grist to the mill.
These things are rarely clear-cut, and no doubt the layers will make money from the situation.
As was seen in the Dutch case
involving Moulinsart, Casterman and reproduction rights for Tintin
, these things can turn on the smallest of details, and take unexpected turns!