Have you two considered family counseling?

My kids started to do that. Whenever I told them to do something, like mop the floor the other one would yell, "Yeah, mop the floor!" So I said "I don't need help giving orders, but I need help mopping the floor. Since you're so eager to help, you do it."
Didn't have to do that more than 3 times before that behavior stopped.
Maybe you can try something like that. Doesn't have to be the same chore.

I think that you guys need to come together on this one and break it down to simple communication. Write down, together, the problem: sd yelling at bd. brainstorm solutions together and compromise then put the decision into action. Even with my two kids that are both bd/bs neither one of them can tell each other what to say or do. That is why they have parents! That is it bottom line. That needs to addressed by both of you - family meeting- all house rules apply to everyone in the house at all times. Rule #1- Do not tell each other what needs to be done leave this up to the parents, rule #2- worry about your own responsibilities- rule #3 each child has their own responsibilities they are…. etc. Hope this helps...

Is your BF right that your bio's tell each other what to do and yell at each other? Does he have any genuine grounds for thinking you have a double standard? Does he fundamentally feel that as an older child SD should be able to tell your youngest what to do, or it is simply that he sees your two bossing each other about and thinks his daughter is doing the same thing and is therefore not out of line?

If there is a bit of bossing going on between your two I understand that that might not rile you the way it does if his daughter does it, but in order to stop it you would need to introduce zero tolerance for all children telling children what to do. Otherwise not only will the children continue to argue the toss over every dispute, so will you and your BF. The rules need to be really clear. To put that across you would need to have a family discussion to clarify that, as 1niccolo suggests, and give them alternative strategies for dealing with situations where they want each other to do something, like ask a parent.

Overall I think its very common for older children to try to assume some kind of authority through seniority, and with girls they seems to do it through a combination of mothering and bossiness. In my experience it does get out of hand if it goes unchecked, and can then be harder to stop because the bossy characters then have a sense of entitlement about telling the others what to do. I have a bit of thing about children telling children what to do because I observed the hierarchy of power among my husbands 5 children and it caused tremendous friction, was often very unfair to the youngest who had everybody telling them what to do, and it gave the eldest an inflated sense of her own role and authority.

I wS thinking along the same lines a Bojangles as I noticed when your boyfriend said your bio kids did it and you never said anything, you response was, "we're not talking about them" which to me indicates he was correct. If he was of course he's going to argue and defend his daughter as she is being singled out and treated differently to the others

I also noticed you thing the baby sitter likes her and you are glad she will be taken away from this sitter when you get married. If you've been complaining to your boyfriend about that to, he's going to feel resentful. He will have all your complaints about his daughter in his head every time an issue comes up with her.

Finally, you said your BD came to you and told you this. Your boyfriend could well consider her to be one of those tattle tale kind of kids who goes running off to mommy telling tales on his daughter. Did you hear the incident, or did you go off on this just because your daughter said it happened. Because if you only had your daughters word for it, you needed to go in lightly on this.

Now, I was not trying to ruffle your feathers there. Just playing the devils advocate. Trying to imagine how your boyfriend might feel, and perhaps make you take a closer look at how you really are handling things. You might actually be the one favouring your own children. Not saying you are, as I said just playing the devils advocate.

As for counselling, if that is really important to both of you, you will make time. Getting the family dynamic and your relationship back on track takes priority over kids activities. But between incollo and Bojangles you might just be bake to resolve this without counselling if you find you actually have one set of rules for his and another for yours, or if you can handle it differently, and do as suggested, "If you're so keen to see the floor mopped, you do it" approach does work perfectly. I've used it. I also had a routine for my three when doing the dishes, one washed and one dried and put away, one got the night off. Didn't matter which one was on drying, every night the cry would go up, it's not fair, I have two jobs, drying and putting away, he/she only has to wash and x isn't doing anything, . I took away the night off, added an extra chore, sweeping the floor. It lasted a week before I was hearing sweeping the floor is too hard. It was, they were little, I had a big kitchen. They agreed happily to go back to the old routine, never heard them complain again about the dishes.

Try taking an objective look at yourself first. You might actually be the problem, you really might not be being fair. No offence intended and I'm not for one second suggesting you are. For your sake though I hope you are because if the problem lies with you treating her differently to your own, it's an easy fix, no counselling needed. You can change yourself far easier than you could ever change your sd or boyfriend. Fingers crossed as I said that it is you. :). Good luck.