The Reynolds’ aren’t really old old blood to the Deck the way the Eichelns are, but they’re old enough to be established and hypercritical of their heirs’ lives, and incredibly anxious if an heir isn’t on hand. I am absolutely certain Elliot is not the first illegitimate child in the family.
Sitting on it now, I don’t think Tomas/Elliot’s line is the direct/head of the family on, as Tomas would have been much more invested in Elliot’s life had that been the case. I think instead they’re descendants of some younger brother of the primary heir a couple generations back-- still close enough to the main line to carry weight, but not close enough for Elliot to be quite as stifled into the primary Reynolds lifestyle. Tomas was largely occupied with ensuring a stronger position both in the dynamics of the family and within the Spades (hence the push for Mariette to have a son) and that continued when El was born. So he left most of the Elliot-raising to Mariette, which, well.
Mariette wasn’t expecting love when she married Tomas Reynolds, but she was expecting him to, y’know. Be around. But Tomas got disinterested when she didn’t pop out a kiddo soon enough (or a spare, after he decided to keep Elliot) and despite being in the same house, emotionally Mariette was left alone in this big house with this colicky little baby and a few helpers (one of whom was the actual mother of this kid she’s supposed to claim is hers and it. It makes her mean. She was already a little mean before she got married, but all of that made her so much worse, and so Elliot grows up with this absent father who only trots him out to the rest of the family on special occasions, this woman he thinks is his mother who is steadily growing to hate him, and his one spot of sunshine is their cook (his actual mother) who lets him hide in the kitchen and watch her make things, but she’s gone by the time he’s 8.
Nadia McAlister was (she died; I’m deciding it now) a really sweet young woman and Three who got a job a little out of her depth when she came to work at the Reynolds’. For a while, things were decent, but once Mariette and Tomas started sleeping in separate rooms it went a little downhill. Nadia was half-seduced, half-spooked into sleeping with Tomas and then suddenly she’s pregnant and terrified she’s going to lose her job. Except Tomas cares about her in the only incredibly messed up way he can, and he can’t help but see this as a chance.
So Nadia gets taken care of throughout her pregnancy by the Reynolds’ doctor and gets an open door policy to stay at the house if she needs it and gets to keep her job even after the baby’s born, and it’s okay. She doesn’t get to keep her baby or tell him he’s hers (she would have named him Michael, after her grandfather, and he has her cheekbones even if he has his father’s eyes) but she sees him every day and gets to talk to him and take care of him the best she can with Mariette watching her like a hawk. Tomas only comes around a couple times after Elliot’s born to try for a spare, but she has better excuses this time and he stops looking at her after a while, so that’s fine. It’s honestly a life she would have been content to bear until Elliot was 15, but then she gets sick and the doctors tell her she isn’t going to get better.
She leaves a letter for Elliot for Tomas to give to him when he turns 15 (he doesn’t; Elliot doesn’t see it until Tomas is dead way too many years later and he’s given what few things Tomas left to him that Mariette didn’t get the chance to burn.) Her last thoughts were of when she said goodbye to her son; holding him tight as she can and murmuring an I love you he doesn’t hear against his hair while he does his best not to cry. Her last words are Elliot and Michael.
Elliot doesn’t remember much of his birth mother, but when he does he remembers a kitchen. A smile. A feeling of being absolutely safe.