I've known my ex since we were first at senior school together - spent most of my teen years around his family, as his parents were far more liberal than most and our group of friends would congregate there after school/college and on a weekend. So my ex's parents have known me since I was 11/12, and his younger siblings grew up before my eyes, pretty much.
Ex and I are now 43. We split 10.5 years ago, when his (unknown-to-me) girlfriend literally gave birth to his child. We have two children - my 23 year old daughter, who isn't his biologically, but whom he helped to raise from early infancy, and a 14 year old son together. We never lived together (my fault because I like my own space, his fault because he didn't want to move out of his "did everything for him" parents' place) but were together. Still having sex, spending time together with both children, talking about growing old together and watching our grandchildren grow up. I had a vague notion maybe 6 months before the split that he was seeing someone else, because he introduced her to our small son. Who told me all about "Daddy's new friend". When I confronted him - and I did, several times, saying "look, if you want to split, just say so... we'll figure it out" - he denounced our son as "a liar" who was just out "to cause trouble" (son was a toddler at this point who - still, to this day - doesn't lie, because his "tells" are just so obvious, it's pointless, really). One day, he took our son out for father/son time - which he did regularly - and then his mother made him call me and tell me "oh, yeah; my girlfriend gave birth to my baby this morning".
He lied to his entire family about our relationship. Said that we'd split a year and a half earlier, that I was causing trouble and trying to prevent him seeing our son, that my daughter's lies (she has severe MH issues) were a big factor... which yes; they were, but I'd done my best to protect my child and him throughout. He said that I was seeing someone else (I wasn't). His younger siblings knew me well enough to call "bullshit", but his parents...?! His grandmother...?!
Of course they believed him and still do to this day. I went from being a part of their family, to them deciding that my daughter and I were both surplus to requirements. And yes; it hurts. But I know that they'll always believe my ex because he's their son/grandson. And that's their problem... because he's a liar who is very devious about honesty. I've done my utmost to facilitate their relationships with both of my children, but they didn't want to see the girl they'd always claimed as their granddaughter, and they're only keen on seeing my son when it suits them - not him (and he's a teenager now, who has better things to do with his time, apparently, than spend it with anyone old enough to be his parent/grandparent).
My advice, OP? Don't waste your energy. They'll believe whatever/whoever they want to... and there'll be nothing you can say or do about it, I'm afraid. 