Daughter and Niece. How would you handle it?

I have a daughter and a niece who grew up together like sisters. They dont live together but they see each other often. Here's my problem, I'm not into disciplining another person's kid because i believe that's what parents are there for right? but somehow i feel like i call on my daughter too much and make her understand her cousin (even if the argument is not her fault at all) to the point that my niece turns to me for help when they have a fight even if it's her fault and that irritates me so much but i don't know what to do... sometimes my in law would see and she would call on her daughter but often times she doesn't. what should i do?

explain to her that you love her alot but you are family and your always gonna be their for eachother. if they argue you have the right to tell her to keep away from your daughter till she calms down or tell her if shes wrong that fighting isnt gonna make things better. let her know it is ok to be wrong but she comes to you cause she trusts you. speak to her and let her know why it was wrong and maybe a way to avoid an argument. its not a punishment it is you care and she trusts you. god bless and good luck

some parents dont know how to teach their kids right from wrong and in my eyes its pretty sad because that is the job of the parents as you said. If i was in your shoes I would sit down with the mother and talk to her one on one and explain to her that she needs to get involved in this as much as you are. She needs to teach her child wat is wrong from what is right and that when her daughter does wrong she needs to confront her as you do to your daughter and explain to her what she did so that she wouldnt repeat it.

Your an adult.
Whether it's your daughter or our niece that need correcting it's your responsibility to do it if the other parent isn't around.
If you keep going on this way, your daughter is going to build up a lot of anger and resentment toward you thinking you favor her cousin over her.
Your niece is still relation to you, so if she is in your house you have every right to discipline if necessary.
You can't be unfair to your daughter just because you feel awkward about disciplining your niece.
If the in law has a problem with that, you two are adults, you can sit down and talk about it.
Be the adult, they are the kids.