I need some advice about my mum please

[YesIamYourSisterInLaw]

Hi everyone,
Right I fear this may be long, sorry.
I'm worried about my mother as she keeps saying things that appear to well be a load of lies. I'm only just getting to an age where I'm realising this and I'm now questioning everything she has ever told me.
We have the same boss who is also a good friend to us both and recently when we have been talking we have realised that the stories just don't add up.
Amongst the possible lies are that my (foster) sister slept with my father because she "remembers things" this always seems to be a theme and it's always after the event that thing come back to her, in this case it was years down the line.
She has often throughput the years told me iv said things I don't recall saying ( nothing even major )
She has told my boss that my aunty abused her and me(I can only speak for myself but I know she certainly didn't when it comes to me) and that I punched my aunty in the face.
I don't know what to do because she has now started saying things about my ex and son which I knew would happen and was dreading.
We are trying to work things out and he came here for a few days last week, I then went back with him for a few day and she rang me while I was there to say that during the week she had been upstairs and then heard our son shouting "no daddy no daddy stop" Ds is 21 months and can only say no out of that whole sentence ( this is the first hole in her story)
She then said when she came downstairs my ex was sat rocking and crying saying I'm a good dad I am over and over.
Ds is with him every other week all week, I have no doubts he is a good dad and I don't believe he would or has hurt him ever.
What do I do? She can't go on like this but I think she genuinely believes what she's saying.
I'm really struggling to come to terms with it as my. Mum has always been my rock, my parents split when I was 9 and my dad was useless, my mum is the only family I'm really close to and now I can't believe a word she says.
Iv got to go out now but I'll check in later and answer any questions

[YesIamYourSisterInLaw]

Anyone?

[Crawling]

I had a friend (ex as well only went out a couple of months but remained friends)who lied. It was done for attention because he had such low self confidance.
He told me his best friend had dies in a car crash, that another friend committed suicide. He told me he had terminal cancer.

I knew he was lying he never had a friend before me aged 17. I ignored the lies which are just a symptom of a much bigger problem.

I worked on the cause I was overly flattering. Gave lots of compliments and encouraged him to try new things. I went anytime he found something he was good at

After two years his confidance grew and he stopped lying. After 6 years he confessed every little lie he told me and burst out crying when I said I know.

He asked me why I never called him out on them and I said because I knew you needed me to believe them right now. I hth it takes alot of time patience and hard work to help someone like this.

[YesIamYourSisterInLaw]

Hi crawling, thanks for taking the time to respond.
Unfortunately I have already told her I don't believe what she's saying and that I think it's all in her head, maybe that wasn't a wise move but I didn't know what to do.
This has been going on for about 10 years now, although it's all fairly new to me as I'm only just at an age where I can start questioning things. I'm remembering things that don't add up left right and centre now.
I spoke to the doctor today and she said all I can do is try and get her to talk to someone but they can't visit her without her permission. I know she won't go so basically I'm just stuck listening to her spin her poison until I tell her to get out of my life and then iv lost my mum.

[Crawling]

Its not to late to try my approach ignore any lies but give extra attention for good behaviour.