I'm at a complete loss for what to do and really need some advice.
Some background:
I was a single mother to DD (aged almost 10)for nearly nine years. Met DP last year and we're now living together. DP and DD get on great. DP has an autistic son (almost 8), who he shares custody with his ex. We have DSD one week on, one week off. DSD has very odd, difficult behaviour. He's very verbal, shouts a lot, stims, argues, can be very annoying and hard to get along with. He has poor social skills and is unhappy at school.
I know that DD finds it very difficult to suddenly be landed with a whole new family, in particular, with an autistic step-brother. I know she dislikes him and doesn't like him being around. I have to admit that I find it a struggle trying to step-parent him and feel very out of my depth.
Anyhow, have found out today that DD is deliberately bullying and winding DSD up, then denying it. I'd had some suspicions before, when DSD came in sobbing that DD was being mean to him. DD swore adamantly that she wasn't and that he'd misunderstood. I got confirmation today that this wasn't the case, that DD had been chanting 'manky monster' repeatedly to DSD at the weekend, having deliberately smeared bird poo on leaves on the trampoline and then asked DSD to move them out the way - obviously he'd moved them, trodden in the bird poo which DD then taunted him about, making him very upset.
She's also asked him to get something for her, then denied having asked him when he brings it.
I feel sick to my stomach that my DD can be so cruel, when usually she's so caring and sweet. She's never done this before and I have absolutely no idea what to do or say to her. I don't want to have an argument or for her to deny it. I know that she's struggling with the new family situation and don't want to make matters worse. 
Does anyone have any advice? Please?