The "extended family is always better than adoption" thing is such a stupid idea. Give the kid to the grandparents who raised their first kid so right that she ended up an unintended teen pregnancy considering giving her baby away? Yeah, sure, that sounds like it'll work great. A mulligan.
It's true that a lot of adoptive parents are shitty, but a lot of that is because adoption agencies will do everything in their power to pretend infant adoption is practically the same as parenting your own kid and that nurture, not nature, shapes most of who your child is. If you're an adoptive parent, you should know that you are, yes, getting someone else's child. That child is likely to in many ways be more like the real bioparents than you, and if it's offensive to you that your kid has in some ways more "real" ties to its biology than its upbringing, you're gonna have a bad time. But there's no adoption agency on earth that will make that really abundantly clear.
Becky's right that kids are the product, not the customer, in the adoption scheme. As such, their interests will never be paramount, regardless of how much lip service everyone involved does to that idea. But Bex's parents aren't the nightmare she makes them out to be, and haven't given up on her or stopped their material support at any time, even when she turned out very different than they, doubtless, were hoping for.
If Becky had been the bio-kid of Mami and Papi, I don't think she realizes life wouldn't have been better. What people now term "mild abuse" was pretty much just the state of parenting in the 80s-90s, and that went double if your parents were immigrants. I don't know any parents of biological kids who would put up with Becky's twitter feed (even if they'd spent some time smacking their kid upside th head when the Berlin Wall was falling and now felt guilty about it).
At this point, as near as I can figure it, Mami and Papi's parenting is entirely governed by the sunk cost fallacy. They don't have another kid to pin their hopes on. Pulling support from Becky would mean they wasted their lives and probably well over a million dollars on less than nothing, a cuckoo who took resources out of their community and faith to leave nothing but pain and lies in her wake.