I'm a twin mum. How old are her twins?
The first year is ridiculously hard, two parents or one. They cry at the same time. Need feeding at the same time. Wake twice as much at night. You have to hear crying constantly.
It's not a competition but don't underestimate the grind for twin parents. It's a whole other ball game.
I am also a twin mom and could not agree more.
I also think none of this is competitive and your sister is probably not in her right mind--sleep deprevation, stress, hormones. Telling her it is great her kids don't have colic is not going to help.
I breastfed, my then husband had a grueling work schedule (medical fellowship), and I lived away from family and did not have a car. I still don't know how I got through the first six months and in the back of my mind was this fear that I was screwing it all up. I went from no experience (and had twins because of fertility treatments) to ALL MY CHILDREN in one go. I felt maybe I had not been intended to be a mom because my body had no cooperated and I was missing whatever magic you needed to do it right. I was tired and scared which is an awful combination.
Try to step out of feeling annoyed that she has it easy from the outside (help from family). We are all lucky in different ways.
Just listen and reassure her that she is doing a good job and this phase will pass.
I think the first three years of having twins was quite difficult, but once they are in any kind of nursery program having twins is WONDERFUL. I am so grateful for the relative ease in planning, scheduling, and meeting the needs of both of my kids.