I'll agree that you can blame some of this on BM, but you have to blame your DH, too. If she has been doing this for a while and he's just now seeing it despite you saying things to him (I'm assuming you have done this, or her brother has), then that lack of parental insight is on him.

He needs to nip this in the bud. She's five. He can easily ground her, take her toys, tell her to stop talking, take her out of fun things you all are doing together, etc. He can also reward good behavior on her part when she follows the rules. Even with only seeing her EOWE, he can still show that in his house, her lying won't be tolerated.

I suggest nanny cams so that if SD makes up a really big, dangerous lie, you have (hopefully) proof to the contrary.

Our BM tells th Skids s***. Used to takes them a bit to adjust to it all. We just have to hold firm on everything.... Structure was what they needed and is what we give. She wouldn’t even take care of them. So they ran rampant and dirty. Have your DH talk to her, and have consequences for when she misbehaves. Your DH should take care of this. I’ll brush the Skids to DH and tell him he’s on kid duty if they start doing s*** because of BM or informing me all the s*** she tells them. He procreated with psycho, so he gets to mess with that. Lol. Took him a while to catch on.

"can only do so much when we get them every other weekend"

If I had a dollar for every time my DH used this as an excuse to not parent, I'd be living alone on an island somewhere drinking fruity drinks with no children in sight. }:)

BM used to tell SD & SS that they didn't have to follow Peri's rules or DH's rule because they were MEAN! It didn't last long because the kiddos got tired of consequences every time they saw us. Was it a hellish time of DH feeling like a horrible person whose kids hated him? Yes! Did it last? Nope. And later on when the kids were in therapy and BM complained that she was having difficulties with the kids following her rules because 'Dad & Peri let us do this!'* and wanted us to agree to force the kids to follow her rules in our home, the therapist's response was simple. Kids can fully understand that different places have different rules and abide by those different rules, otherwise all children would need to be homeschooled. Young children in daycare learn quickly that daycare may have different rules than home. Kids are smarter than adults like to give them credit for. But kids are kids and will take advantage if they think they can get away with it.

*The kids also tried to do this the opposite way: telling us 'Mom lets us do this so you should too!'. We always responded with "That's nice. Our house, our rules. Mom's house, mom's rules." They don't try that sort of thing with us anymore, though apparently still do it to BM occasionally.

"He can only do so much when we get them every other weekend," is not in it's own an excuse.

SO's little one is aggressive, sleeps in moms bed, doesn't eat at moms. Guess what happens at our house he has consequences. As a reaction to those consequences SO's little one doesn't get aggressive, doesn't sleep in our bed, and eats what he's given when he's with us.

Did our rules change what happens at BM's, No, but they did change what happens at ours.

If DH had from day one set the rules "no lie" and stood his ground you might see a completely different child. Countless stepfamilies can attest to the fact that children can follow completely separate rules for separate houses. They can be completely different children.

Heck 'normal' families see this when the child is with grandma / grandpa or even at school.

Even just depending on the adult that handles them.

There are countless step mothers here who talk about how their step child does good with whatever they require until daddy gets home. Suddenly stepchild doesn't have to listen and it's daddies fault for not working with his partner and backing her rules.

Yes every other weekend means only so much you can do on some things. You can't change school grades when the child only does homework every other weekend. Your child isn't going to drop 20 pounds and not be diabetic if BM's still feeding them soda and candy 12 out of 14 days.

But you chance influence the behavior in YOUR home and children CAN follow rules even if BM doesn't make them. She's 5. Find her carrot and use it against her.

I wish I had the advice from the other stepmoms on here when my SD was 5 and causing drama and telling BM on us (me and SO) anytime she didn't get her way at our house knowing BM would cause a major shit storm over it. I would have disengaged back then! Instead I tried staying engaged for several more years and then I found this Godsend of a site. Yes, BM influences your SD but your DH needs to be the one at your home parenting. Disengage...if DH gets upset oh well. I know mine doesn't like it but I don't care it is his job to parent his daughter. Refer her to her dad for EVERYTHING and have daddy explain that BM rules/expectations are for BM's house. BM does not control your house and here are the rules/expectations for your home. Trust me, if daddy doesn't start parenting and continues to let BM and SD rule it will be MUCH worse in the future.

Disengagement doesn't have to be as black and white as "fully engaged 'bonus' mom" and "showing zero interest in the child and pretending they don't exist". There is a lot of room in the middle.

You're already partially disengaging by redirecting parenting to your DH and demanding he handle her behavior.

You can set limits on what you will tolerate and disengage when that limit has been hit. If you enjoy doing puzzles with SD and she behaves while doing them, then do them with her. However, if she gets pouty or you find out she's lying about puzzles (eg telling BM you make her do them and punish her if she doesn't), then no more puzzles. If she asks why, tell her it's because she lied and you won't play with liars. Then walk away.

Disengage as limits are hit and maintain your disengagement until behaviors change for the long term (assuming you ever want to re-engage). SD complains about the food you make? DH can make her food and serve it to her. She throws a fit in the middle of a game? Get up and walk away without a word to her. She lies about things you say? Don't talk to her outside hello and goodbye. My guess is if you slowly disengage and tell your DH every time a limit is hit, he'll tolerate it much better than a proverbial "dumping" of his kid.

I say this to you because you don't want to upset your DH. You can totally 100% disengage from SD today and you'd be well within your rights to do so. Your DH doesn't have to like it, but you have every right to be respected by individuals interacting with you and dictating what kind of relationship you have with them. You can't forbid SD from coming in your home (at least not given her current behavior), but you certainly don't have to help with her care or subject yourself to her poor behavior. If your DH doesn't like it, then he can fix problem or risk losing his wife. His choice.