B.C.
If your sister was the only person who had any complaints - then you pleased %99.99 percent of your guests/family.
Woo Hoo!
Pat yourself on the back and do your happy dance!
Your sister didn't have to complain - she did.
Your parents didn't have to listen - they did.
Your parents didn't have to pass on said complaints - they did.
Your mom in her 70's offered to take back Christmas to appease ONE person - that's a whole separate therapy session right there - at least that idea got tossed out the window eventually.
Why is everyone so willing to bend over backwards, wiggle their ears and whistle Dixie trying to please this one person who can not/will not be pleased no matter what anyone does?
Just accept she lives/loves to complain and isn't happy unless she's unhappy.
Accept her as she is - but for goodness sake - don't take her seriously - nobody should.
So short answer - let it go!