So this woman is telling lies about her husband's family, painting them as really awful people, presumably to get attention - and nobody thinks this is something that requires addressing?
For someone to go to such lengths to maintain a fiction like that, surely indicates that something is wrong with them?
I am taking the OPs story at face value. That the sil is telling lots of people on the net a load of lies, presenting all these people as terrible people doing awful things to her - and I am accepting the OPs assertion that none of this is true.
This leaves the SIL a what? total fantasist? Why? Does she have a problem with them all? Does she hate them? Does she believe what she is saying to be true? Is she an attention seeker, looking to the net for lots of sympathy? Is she a troll on the forums she is on? Is she saying these same things to her friends in rl?
It is not normal to do this. It is not normal to pretend that your lovely MIL is a cow, to say your bil is an alcoholic, to say your sil brought on her own miscarriage (
that one is evil) accusing them of fraud, alleging violence against her children...
I am a bit surprised that the response to the OP on this and the other thread (which I remember) is to mind her own business. [boggle] If massive lies are being constantly told about you - how much more your business can it be?
There is clearly a huge problem here! And it needs sorting out.