In the past it was quite common to 'hide' an illegitimate child by saying that the actual mother's mother had given birth to it.
Right up until the 1970 it was consered a shameful thing to have had an illegimtimate child. This is one of the reasons so many were given up for adoption.
My MiL taught in a rural area and she often talked about the number of children she taught who had mothers who were women in their 50s. Of course these women had not given birth to these children, but, probably, one of their unmarried teenage daughters was the parent of the child and to stop the girl bringing disgrace on herself and the family by admitting to an illegitimate birth, this was the way round it.
Tragically, a number of these children were the result of crimes of incest, where the girl had been raped by a family member, often their father.
In cases like this, as in your family, Cariadagain I would let sleeping dogs lie. You do not know what other family members know and who has no idea.
My grandfather was illegitimate, but he had a cover story based on his father dying before he was born. no-one in the family knew, except, possibly his eldest son - and one daughter knew this was not true. My grandfather had 11 children.
In the early years of this century, there was a family gathering at my parents house and i was there helping with the catering and a cnversation started about my grandfather's origins, and someone brought out the story we had all been brought up on of his father dying before his birth, when one of my aunts suddenly said, actually, I saw his mother's death certficate - and she was described as a spinster.
The response from his children, all in their 70s and 80s was interesting, several said, 'Yes, I had suspected that,' but several were taken entirely by surprise. No one was bothered or upset.
So, Cariadagain some of your relatives may have realised what happened and that relationships between different people in the family are not as presented, but many may genuinely not realise. I mean you have only just realised. So, as I said, let sleeping dogs lie.