I think it's a sad thing if you aren't the 'norm'. If you think about it, how we are introduced to ourselves and the world around us during the first 10-12 years is through our immediate family (after that peers matter most so hopefully the family set us up good and relatively prepared!). If one's introduction is being cursed at or otherwise verbally abused, this is how you view the world, and yourself. You know all those grumpy and unhappy and mean people we've all encountered from time to time...maybe their view of self and others had a rough start. You only get one childhood.
For those of you who vent from time to time with your kids...you are right, it happens and is human. Forgive yourself and know it might happen again and again. Normal and ok. BUT it is YOUR obligation to repair whatever unintended damage may have occurred while you were frustrated. Just as we teach our children to "be nice" and "get along". If they don't or can't it is our JOB to help them repair. Otherwise we are setting them up for many more and worse interpersonal conflicts.
I might suggest in your sisters case she is unhappy and has some unresolved issues that make her so angry. Wait til her kids hit adolescence and you will see what her parenting does...either through poor behavior with others, poor self esteem, eating disorders....the list goes on and on and as someone with extensive experience with high risk youth, I guarantee parents like you sister ensure I WILL ALWAYS HAVE A JOB.
Your sister is over burdened, undersupported, and very unhappy. It makes my heart hurt because she is missing out on the beauty of her childrens development and the beauty of parenthood. She should seek counseling to figure out how to find joy and how to manage her anger...maybe even discover WHY she's so angry because I guarantee it isn't the kids fault. Why have kids if when they act like kids you get pissed instead of recognize the learning opportunities? Take a break and if that doesn't work, look inward.
Keep doing what you're doing and model to your sister. If she finds humor in belittling her children, belittling love, and belittling respect....she must be fine when her husband or boss or sales clerk or post office employee or driver that cut her off and gave her the finger...because all those things are acceptable and funny too????
Negativity towards anyone is more of a reflection on the instigator than victim and in my view, your sister and parents like her are bullies. Real tough...picking on children. Keep it up mama's who endorse, accept, tolerate, and participate in this abusive behavior. Keep it up. Because of YOU I get to help homeless and runaway and foster youth discover their value and learn how to thrive DISPITE their parents. I get to feel the reward of seeing your children excel better without you. I get to watch them flourish in education and relationships through full awareness of how NOT to treat them selves and others based on how their parents treated them. I get to see resiliencey.
You know what you get? A life without your adult children to spend X-mas with.
Sorry. I'm done. Just take L.'s lead and love your children. Please.