step-daughter

My 6 year old step-daughter has been manipulating my husband to do things. It's become so upsetting especially now that we have a 3 year old. She's very mean towards him and tries to get everything he wants. Like for instance, today, I came home after running errands and I asked both kids to clean up their mess. My son cleaned it all up and I kept asking her to help she then said, "I didn't do it, it was all him." I then said "come on I know it was both of you so please help him out." She then said, "no I wasn't even here." (Which was obviously a lie and my husband laughed thinking it was funny.) then I started making lunch, and my son asked for the tablet and she then said "no you can't have it because we are going to fight for it."  Couple minutes later I find my husband and her in his office talking about her using it and my son using her own phone. I then opened the door and she shut it on my face. I was really upset. I then peeked again looking at my husband to discipline her or something and he did nothing. (I was furious) Then I asked what they were talking about and she said nothing leave and I said what's going on and he told me to leave because he's talking  to her. I felt like a dumbass because I never get included and she knows she can get her dad to do anything for her. I feel like I should get some say in this or be included so she knows I make the decisions too. You know? At the end, she got the tablet and my son got her phone. Even though he asked for it firsthand and even though he deserved it for cleaning up everything. He didn't get it. I need help/advice idk what to do any more 😩 I just want them to be treated fairly.