This is probably the worst thing MiL did. Certainly in my book.
When I met DW initially, we had a wobble, and split up. During which she saw another guy, and became pregnant. He became very abusive, and she quickly had to get a restaining order.
In the meantime, we reconciled, and I left my home town to move in (we had met as penpals - very old school). I was with DW for the last 8 weeks of pregnancy, and the birth, when DS was born. DS natural father just vanished into nowhere (well, nary a squeak).
For various reasons we (in hindsight, foolishly) never really had a plan for addressing DS parentage, and he grew up knowing me as "Dad".
Until about 2011, when MiL suddenly announced she'd heard from DS natural father on Facebook, and he was demanding to meet DS, and if MiL didn't sort it out, he'd turn up one day.
At this point, neither I nor DW wanted to be on Facebook, but DS was. So we had to trust to the messages MiL showed us.
This forced us into having to reveal the truth to DS in the least planned and most rushed way possible. (After he learned he cqame up to me, hugged me and said "no one else is my Dad."). However, he did go to meet his natural father, who called round to pick him up.
Because I'm who I am, when he turned up, I insisted he came in (I had checked with DW beforehand, and she was OK, as I was there) as "I don't talk on the street". After he'd seen DS a couple of times, he dropped DS when MiL and her DP were round for a cuppa. He was a little nervous seeing MiL (DW said he hated her ... maybe he spotted what she was really like ??????) but sat down. He then insisted on saying that he was really proud that DS had clearly turned out to be a credit to us both, and that "it takes a man to raise man", and he wanted to shake my hand ...
(He pretty much fizzled out after that - possibly because he had a wife & subsequent kids - but more likely because he seemed to have no interest in DS).
So far, so stressful, but shit happens.
Except, DW and I since found out that it was MiL that contacted DS natural father, and egged him on by telling him how bad a father I was being. It was she that faked messages to appear from him which pressurised us into our course of action. The only bitter smile to emerge was DS natural father completely wrong-footed her, and subsequently ripped her a new one over her deception.
We would tell DS, but he's still close to his "Nan", and too under her spell. Which makes our relationship with him a little delicate. We daren't let slip anything that circumvents the NC. (Despite the countless times someone in the family has "got cancer" - yes, that trick too).
When I moved in with DW, MiL promised she and her DP would regard DS as mine. The reason I have decided there can be no forgiveness is after she assualted me (as I said previously police were called) and her DP had to literally drag her out of our house, she was screaming about how I was not DS real father, never had been, would never amount to anything, and that DW should check with her counsellor, as "she knows".
Finishing on that note. I said previously that MiL had tried to accuse me of all sorts to DWs counsellor. (Turns out it's not that easy. Thank God). When DWs counsellor told DW, DWs first instinct was to "go ballistic". This was before we had discovered Mumsnet, so couldn't see what was happening as a pattern, or trait. However, I couldn't believe anyone could be so horrid, and wondered if might be due to some sort of mental illness, or alcohol (because MiL does put it away ...). Up until then, we'd always though MiL "memory problems" (she never remembered saying or doing certain things ...) were genuine.
So by letting slip that she not only remembered calling the counsellor, but exactly what she had said, she rather confirmed she was in full possession of her faculties, and knew exactly what she was doing. I know it's not scientific, but it does weigh in with PPs questions about whether they know they are doing it.
As I say, this all happened pre-Mumsnet. It's only through this site that we realised we weren't alone, and that there really are some nutters around.
Sadly, for DW, this realisation has meant she's had to revise a lot of experiences from childhood. But, reading these forums, as she says "a lot of things make sense now".
I know a lot of people say "you have a DP problem, not a MiL problem". However MiL was so pitch perfect, that even before we'd met, DW insisted her DM "was her best friend". Which says a lot about how friends fuck off the second you get diagnosed with a lifelong illness and start using a wheelchair.
I'm guessing that a guaranteed way to become the target of a narc is to refuse to let them get their way in something ....