Step daughter is telling her mum lies about our life

[Weddingplanningandlovingit]

My step daughter to be reports everything that is said in our home back to her mum. I know this as SS tells us. Not only this, she makes up arguments that we haven’t had, making DP’s ex very happy (again from SS). She’s a teenager and I’ve been with her dad for four years. I wasn’t the OW, they were separated when we met. AIBU to challenge her on this?

then maybe it is time to lay it all out.and play it by ear with her reaction.
start with sitting down an telling her this isn't about telling anoyone off or making an argument.but about wanting to no why she felt the need to say these things and what can you do to move thing forward and get to a better place with eachother.
don't make it so your having a go at what she is doing.and more on what is she feeling and whats missing that you aren't perhaps giving her.
my auntie once wrote it all down on a letter to my teen cousin.and put it outside her door with a note and some chocs saying "Hopefully these yummy chocolate will ease the nature of this note luv"

Explain to boy that his sister is finding it difficult and she will settle down. Does he think mum believes every word? Can she have more time alone with dad? Try to ignore it, what are you doing that mum can't know about? It's irritating but part of situation you are in.

Came to this post from another post of yours. This sounds very difficult to handle .. I disagree with people telling you that you should take the high road as if it’s easy .. I can see you are struggling.

I would say don’t confront her or do anything without ur dp knowing what you will do /did. But if he isn’t able to do something about it, then write down some bullet points of what you want to get out of a small chat with her ( I know you might feel negative and sensitive ) and agree with your partner on those points . Go to speak to her in the presence of her brother .

Maybe don’t get Dp directly involved as it will make her feel angry if he takes ur side no matter how right that is.

But in general , let ur dp lay the boundaries for the future.

Look, likely ur dp has wronged her mother and he wronged him before they realised they’re not suitable anymore to be married.. the child grew up seeing her mum cry, and so naturally she is struggling to see her dad making another woman smile while he couldn’t do that to her mum...

She is taking it personal and probably wants to make her mum feel better.

It’s a crap situation for u... it doesn’t have to be this way. U need to stop her from continuing to take out her hurt on u and ur life ... but u must be empathetic enough to be tactful