you are correct!   Unfortunately when you live with a child, it really hard to leave alldisciplining to the Bio. Like if child talks back to you,  you are going to say, wate a minute, I have to tell your mother about this ! No, there certain things you must handle 

1. You need to get on the same page with your partner about what is disrespect and what isn't. I'm pretty lax in some ways and not a big believer in hierarchy, etc. So what I view as disrespectful may be entirely different than what you do - and what your partner and her child do. None of these views are necessarily wrong but what is wrong is having expectations without communication. However, the only way to get on the same page is to talk -neutrally - about what kind of behaviour is seen as respect. For example, some people in my childhood had parents who expected to be called Sir and Ma'am. This is not something that I feel needs to be said to be 'respectful', but others may do. As long as everyone knows what the terms of respect are and they are consistent, it's fine. What are your terms of respect? What are your partner's?

2. It's not your job to protect your partner from her 8 yr old child - unless it's extreme. I love my partner and sometimes I get upset about the way his kids treat him (pretty rare, but it happens) I feel the need to protect him. He does not WANT me to protect him from his own kids. You can have your own expectations about the way you want to be treated, but she may have different expecations and as long as it's not horrible - that's ok. 

3. Every family is its own culture and when you blend families you create culture shock. For example, BM demands that her kids ask permission to leave the dinner table. My partner doesn't 'undo' that - he doesn't demand it, but he hasn't told them they don't have to do it - so they do it in our presence. I never expected this of my son. He found it weird and oppressive, I too found it a bit odd. But he does it now too even when my partner and his kids aren't here and it's kinda nice so I don't 'undo' it either. BM and my partner allow food outside of the dining area. I do not. No food or drink (except water) upstairs at all. That's my rule. His kids found it weird, but they abide by it now. But it had to be communicated. I didn't get cross when my OSS 'broke' the rule - I said you didn't know, now you do. He told on YSS, I said he didn't know and I didn't catch him. No one is in trouble. But you will be next time. 

If you are fairly new to stepparenting, hopefully that means you aren't super invested in this relationship yet. Do you live together? If you are having problems with his attitude now, just wait until he's 13/14 and mommy dearest still refuses to parent him.