Ah 'difficult' - all too familiar with that label!
Before I got married and had children - parents and sis decided that they would do a family Christmas away. So booked a four bed cottage without consulting me. Then told me this was the plan and sis and I were to split the cost 50:50 (I was to have one bedroom (the smallest, as it was just me), sis have two for her/her DH and kids, and parents the fourth one). Parents to pay nothing as they were helping sis out with childcare (eh? isn't that called grand parenting when parents are there too?).
I said NO! My share was about £1K (and I dare say I would be expected to pay for half the food too) and at no point did they decide to ask me about it before booking! I stayed at home (my house - moved out at 18) and saw friends. I was being VERY difficult apparently and fielded lots of angry/tearful phone calls at the time.
Sis is the golden child - I was the defiant one (although I wasn't badly behaved). I think I surprised them a bit by ending up with a job paying significantly more than Dsis - hence the expectation that I pay above my fair share for their ideas.
And so it continued for another decade...lots of anecdotes. When they started doing it with my kids...I called them out on it. Denial. Gave up and drifted away (which was very easy - we just stopped inviting them) - helps that I have a very supportive spouse who can see straight through them too. Still hurts though 
. I just do the bare minimum now as NC would cause too much angst.
Anyway, yes - you can do LC with dignity. Just slowly start making less and less effort. Keep conversations grey rock. Avoid asking them for any favours at all.
What is your relationship like with your Sister? If it ok - then I would make more effort that but completely avoid talking about your parents.