In a viral post shared in the Am I The A**hole group, which can be seen here, user Hot-Pineapple-7873 explained she adopted Lucas when he was 3 years old.

The Reddit user, 38, added Lucas - now 17 - was the biological son of her older sister Jenny, 41, and her husband Derek, 42.

She said the couple did not want to start a family when Lucas was born and that he lives with a “chronic condition” that affects his mobility, meaning he must use a wheelchair to get around.

The Reddit user added: “Jenny and Derek said that having a child in a wheelchair wouldn’t fit into their ‘image of a dream family.’

“They were considering surrendering custody of Lucas, but I decided I would adopt him. Jenny and Derek signed custody over to me and I became his legal parent.”

She added that her sister sent money over to cover Lucas’s expenses until they had their two children, now aged 14 and 12.

The Reddit user continued: “Lucas never expressed an interest in finding out who his biological parents are, I made it clear to him that if he changes his mind it won’t hurt our feelings and we can talk about it.

“Recently, Lucas showed me a Facebook message he received off Derek. I was shocked. Derek dropped the bomb on Lucas that he and Jenny are his ‘real’ parents and that I adopted him when he was 3.

“Jenny and Derek never discussed telling Lucas about this with me, so to say I was angry was an understatement. I called Jenny and asked her what the hell was going on. Jenny brushed it off and said that Derek was drunk and sent the message on a whim.

“I told Jenny that what Derek did was inappropriate and inconsiderate of Lucas. She said that Derek can do what he wants and I can’t shield Lucas from the truth.”

The poster then asked other Reddit users for advice about whether to tell Lucas about the reason his biological parents gave him up.

Since the post was shared Saturday, it has racked up 6,100 upvotes and 599 comments.

One Reddit user, whose comment was upvoted 10,300 times, said: “Wow, first off I want to give you props for stepping up to the plate. What a challenging situation to handle.

“I’m adopted and never ever wanted any info on my biological ancestors, but the fact that his aunt and uncle are biological parents is a whole other dynamic.

“I would consult with a family therapist and perhaps you and Lucas can meet with them. NTA [not the a**hole].”

Another added: “I agree that Lucas should be told. You and your husband should do that only with the guidance of a therapist. Lucas should also have therapy to deal with any issues that may arise.

“You should also cut off all contact between your son and Derek and Jenny. At least until a therapist recommends differently. NTA.”

A third posted: “This is information that is best coming from OP [original poster] and in a loving, supportive environment, like a therapy office.

“Letting him go to meet Derek and hear whatever story Derek is going to tell before he knows the truth is setting him up for bigger issues. OP should get out in front of this. It’s time.”

Newsweek has contacted Hot-Pineapple-7873 for comment.

According to the Adoption Network, some 135,000 children in the U.S. are adopted each year.

It added that while no more than 2 percent of Americans have actually adopted, more than one-third have considered it.