This Is Us : Rebecca lying to Randall

I can totally understand her not wanting drug-addict William near her infant son. I can even understand her concerns about clean William trying to reclaim her 9 year old son and that she thought it might be confusing and difficult for him.

What I wouldn't be able to forgive, as Randall, is her keeping it from him after his 18th birthday. He would have been an adult, and more able to handle it, and also the issues of losing custody would be moot.

The only reason to continue to keep the information from him at that point was purely selfish - she knew he'd be very angry at her for lying for so long.

I agree, it was a really bad decision to keep it from Randall once he turned 18 and was mature enough to decide for himself if he wanted to pursue a relationship with William. Adoptions can be emotional minefields. My best friend and both of her siblings were adopted from three different sets of biological parents. Their adopted parents gave each of them their blessing to find their biological parents once they turned 18, and there were three different outcomes. Both biological parents of the oldest sibling welcomed her with open arms and she has a good relationship with both of them. The biological father of my best friend welcomed a relationship with her, but her biological mother did not. Neither of the biological parents of the other sibling wanted anything to do with him. They even refused to answer simple health history questions for him.

And it will be even worse when William dies and Randall realizes he could have had more time.

It will be interesting to see if the writers come up with anything satisfactory to try to explain this. They are constantly revealing twists that shed new light on situations while we are left to understand and judge with only the information they have allowed us to know. Cool and frustrating at the same time!