If you've allowed it to go on for 6 years, that is a problem. You need to talk to him in no uncertain terms, not 'ask him about it'. What happened when he said 'don't start that s*&t again'? Did you back down? Find a way to fix this, because it's harming your daughter. Some counseling would probably help. First you alone, then you with your husband. It's time to draw your line in the sand, or this will never stop.

There is an issue here if he is disciplining your daughter and not his.
Next time it comes up during dinner you clearly state, not ask him , clearly state it " THIS SPEECH IS FOR BOTH OUR DAUGHTERS PERIOD'
You will not lecture my daugther about finishing meals from this day forth when your daughter does the same thing. If you continue, then i will lecture your daughter on not finishing meals as well. Would you like that? Is That how you want our relationship in parenting to be? So stop this and do not tell me 'oh dont start this sh*t again.' They either get treated equally or you do not discipline my daughter from this day forth.My opinion, he should not discipline your daughter.Period! Do you discipline his? Have you allowed him to do whatever he feels like with your daughter? If so, then this is an issue, you must both sit down and tell him, disciplining of your daughter is causing resentment. From now on, you will discipline your daughter and he will with his.
He maybe more harsh on your daugther because she is more wild and wants to control her. That is not his job.
How do the girls get along? How does your daughter feel about her stepdad? Have you had a one on one talk with her lately?
What makes me uneasy if the fact you say its been going on for 6 years. So he has been like this from the beginning. Why? have you told him to back off? Tell him to stop disciplining your daughter period. Its causign more harm than good.

Since both girls are the same age, it seems reasonable that they should have the same rules and limits, and if the rules and limits are to be truly effective, they have to be enforced all the time, consistently.

Obvious, huh? Stating this may be a way to broach the topic of discipline in a general way that helps resolve things constructively. If you can then discuss parenting and discipline strategies unemotionally -- without alleging favoritism -- then you may be able to make some improvements.

Alleging favoritism is a hot button. You may be right -- but it'll shut down a productive discussion before it can even start. Your husband will be offended and angry if you accuse him of being unfair -- So don't do it! Avoid putting him on the defensive and remain calm at all costs. You may even want to admit that you know it's your daughter that's more of a handful, but state that you're looking for discipline strategies that you can both agree on and both implement consistently.

This may be a case when talking with a child therapist can help. You do have two girls with very different temperaments, so finding an effective, loving, consistent discipline strategy that works for both will be a challenge. But they do exist. And if you both can get on the same page about what the rules will be in your household, and how rule enforcement will be handled for both girls, then things will begin get better.

In general, I tend to agree with the "each parent disciplines his/her own child" camp, but since your kids were infants when you two married, I think this may call for an exception.

ASSUMPTION -- I just realized - I'm assuming both girls are living with you two full-time? If his daughter is "just visiting", then I'd go back into the "each parent disciplines his/her own child" camp.

Because if his daughter lives with her mom most of the time, he could very easily be starting from a "guest mentality," which has a totally different dynamic. If he has limited time with his daughter, he's not going to want to "spoil" that precious time by disciplining her. And you'd need to acknowledge the reasonableness of his feelings about not making his daughter's time there unpleasant. You could then use that as a springboard into a discussion about the difficult position that puts your own daughter in.

But given that this issue has already become contentious, I'd have the discussions with a child psychologist. Your DH needs to reach his own conclusions about disciplining his own "visiting" daughter appropriately. Once he's on the way there, then it's time to work in a way to make it 'fair' to both girls.