Andrew was supposedly an anonymous sperm donor. A little while back JJ told Dex the story from the old diary that he found from his mum's friend Phillipa. I can't remember all the details, but I think Cara & Remi were going to use a sample from a US donor, but Cara instead actually used a sample that her friend Phillipa had, but didn't use (without telling Remi). So, Cara & Remi never knew Andrew (I am not sure whether Phillipa actually knew Andrew, or he was an anonymous donor for her as well - maybe someone else remembers what JJ said about what was in the diary??).
Some of it is mentioned in this Metro.co.uk article:
Neighbours spoilers: Andrew Rodwell destroyed as JJ Varga-Murphy drops huge bombshell
https://metro.co.uk/2023/10/22/neigh...o=newsnow-feed
Relevant parts of the article include:
"JJ, as viewers know, believes that Andrew is his biological father, having discovered an old diary belonging to his mum?s friend Phillipa which pretty much confirms such a theory."
And:
"It?s likely that Wendy has no clue that Andrew anonymously donated a sample of his sperm to Cara. Cara, meanwhile, is thought to have let Remi believe that the sample she used to conceive was from a US donor."