Ok so bare with me on this!

I am a step Dad (not married) and have been with my Mrs and her two girls (13 and 18) for about five years. 
 

Generally things have been ok in our relationship and we never argue. However there are times when I don't agree with the way her girls speak to her. The eldest can be very rude and dismissive in her mannerisms towards her Mum and essentially talks to her like a piece of dirt and now the 13 year old (who essentially wants to be like her sister) is beginning to act in the same manner.

when I raise this with her (Mum) she just brushes it off like it's nothing however will not let me tell her daughter off and I find myself biting my tongue.

My Mrs's ex husband left her and their kids for another woman and since then she had blamed herself for it and subsequently spoiled her kids giving them everything they want and letting them do whatever they want (to an extent) without laying down or enforcing any boundaries. Don't get me wrong, to everyone else they are extremely polite and well mannered but their Mum just gets treated like a doormat and it's killing me to watch the woman I love being spoken to like this.

Recently the youngest has begun to push her boundaries with me (answering back etc) which is to be expected at her age. The problem is that my partner refuses to acknowledge this and leaps to her defence when I express my annoyance at it. I work very hard and on top of that I clean the house, cook etc as I view our relationship to be a partnership.

The girls aren't even expected to make their own beds in the morning, their bedroom is a tip, they are not made to do any chores except for washing up occasionally and are generally lazy.

I resent working as hard as I do picking up after the kids like I'm their personal slave only to be cheeked by them and not supported by my Mrs who defends their actions. As I said we have a good relationship but if anything will drive a wedge between us it will be this

Thanks for baring with me on this. Some advise would be appreciated!