SD lies, BM covers
Im new here because I feel like I have no where else to turn. SD12 has been a liar for as long as ive been around (about 3 years now) its gone from very minor things to huge things. We've spent countless hours letting her know lying is NOT okay. We also sat down with BM recently and she revealed a lot of lies SD has told. So we were happy to be on the same page of getting SD to stop lying. Well Thursday SD asked if she could go back to BM a day early we said ok. Then I see SD posting selfies on Instagram all day Friday come to find out she stayed home from school. Fast forward to today SD missed school again. So I messaged her and asked why she missed both days. She didnt reply then BM texted DH almost right after saying SD was sick. I texted SS14 and he says SD wasnt sick and just didnt want to go to school. So BM is covering for SD! What can we do? How are we going to stop her lying if BM enables it?
You aren't going to change it
You aren't going to change it. My SS gradually became a very good liar, thanks to BM's role modeling, and he's still one, at 19. I don't believe anything he says without independent verification.
I don't think there's a whole
I don't think there's a whole lot you can do. What we've done with SD7 (much younger than 12) is to remove all benefit of the doubt and sometimes, for effect, show doubt even when we know she's telling the truth. We'll say, "we're sorry, we'd like to believe you but we just can't because you've been telling so many lies lately. No one will believe what you say if you continue to lie." This is on top of consequences when we catch her in a lie. We don't know if this will work or not - it sounds like it has not worked for you. The good news in our case is that BM is not a liar (she was a cheater, but she never lied about her cheating), so this is not behavior that is being modeled for the SDs in either home.
Call the truancy office and
Call the truancy office and report the SD/BM bullshit.