You did it! We figured this one out! It is a truth universally acknowledged (by me for certain) that the Bitchery pretty much knows everything, and really, it's true. Scroll down to see the solution for this HaBO - and many thanks!

This HaBO is from Tammy, who thinks the book she’s looking for is a Harlequin Presents:
I am looking for the title of a Harlequin Presents I read several years ago.
It starts out in England. This father, who is either Brazilian or Spanish, is looking for his son. His girlfriend/wife took off with their child, and she stole a bunch of stuff. When she returned to England, she told her family a bunch of lies about him. Both the mother of the baby and the aunt are adopted. The parents spoiled the mother of the baby and generally ignored the aunt.
The mother dies, and her sister is taking care of the baby in England, where the father finds them there. The aunt tries to explain to him that his son has severe emotional problems. He clings to the aunt constantly, cries all the time, and won’t go to sleep. The father thinks i’s an excuse and plans to leave with his son the next day.
The next day, he discovers she wasn’t exaggerating. Not only does his son not know him, he is terrified of him. The baby is two or three years old. The father tries to help the aunt out by cooking, doing wash, whatever in the hopes of winning over his so, but after several weeks nothing improves.
He decides they are going to Spain/Brazil while the baby is asleep. He takes them on his private plane, and when the baby wakes up, they are in his new home. The father has warned the staff not to look at baby, or talk to him. Everyone thanks the aunt for bringing the baby home.
The father and aunt have a deal: if the baby won’t accept him, she can take him back to England. It is breaking the father’s heart. He runs a rehab center and designs prosthetics for disabled children. There is a female ‘cousin,’ who he is close to. The baby’s mother told the aunt the father was sleeping with her, but it is a lie as the cousin is a lesbian.
Daily, the father tries to win his son over, but nothing. The aunt is torn. She has fallen in love with the baby, and if he bonds with the father, she will have to leave him there and return to London. But she can see how much the father is suffering. The father’s mother begs her to help him and the aunt does. They start hugging and holding hands to show that if the aunt likes him, then the boy will.
The village throws a party for the baby to welcome him home.
The last step to the father bonding with the baby is for him to play with other little boys in front of his son. When his son sees them having fun, he gets jealous and wants to play.
The father asks the aunt to marry him, but she thinks he only does it out of a sense of responsibility and to keep her there because his son loves her and considers her his mother.
There is an epilogue:
The father is celebrating a milestone birthday, and he think everyone has forgotten. But when he comes home from work, he sees all these children and adults coming out from the pool house and the garden. It’s all the kids he has helped over the years. His wife (the aunt) invited them all over to help him celebrate his birthday.
Ig you can help me find this book, I would be forever grateful! I loved this book; it was so sad, yet so touching.
There’s a lot of detail here and I’m positive someone knows it.

Could it be “The Kristallis Baby” by Natalie Rivers?
https://www.goodreads.com/book/show/1296366.The_Kristallis_Baby?from_search=true
Could it be “The Kristallis Baby” by Natalie Rivers? Some of the details fit (but I realize that Corfu is not in Spain…)
https://www.goodreads.com/book/show/1296366.The_Kristallis_Baby?from_search=true
Goodreads has a list of books featuring aunts/uncles raising their orphaned nieces/nephews:
http://www.goodreads.com/list/show/98223.Aunt_Uncle_Caring_for_orphaned_Niece_Nephew
Oh no, I think I read this one, and I have no idea what it is. Wish I could be more help, but basically all I’ve got is reassurance this wasn’t just a fever dream. I *think* the father was Brazilian?
I don’t read a ton of Harlequin, so Sarah Mayberry or Sarah Morgan, maybe? Or Julia James – I read some of hers before realizing it wasn’t the same author as Julie James. I also just randomly try things sometimes though so….
I know I read this in the past coupla years. In fact, I am positive it was SB recommended
Sadly, not the book you’re looking for, but Worth It All by Claudia Connor features a hero who designs prosthetics and also a small (girl) child. I read it when it came out about a year ago and enjoyed it. The little girl belongs to the heroine, and the meet-cute is an interaction that the child and the hero have at a diner when the child sees the hero’s prosthetic and then shows him hers.
Hi all! Tammy’s found the book. It’s The Italian’s Demand by Sara Wood!