Rude children without manners!

I love my niece and nephew but my god they are 8 and 10 and have NO manners, NO respect for anyone, and just whine about what they want over and over. The vast majority of interaction between the 2 of them is fighting and whining. They have had very little discipline and it shows. I will not tolerate it but I am not their parent and therefor it is very awkward when they are being outwardly rude towards me and their parent says/does nothing about it. I’m at the point where I ignore them when they are misbehaving and have to distance myself because I want to yell at them. I would be soooo ashamed if my children behaved that way. How do parents not get it??!!!