I would dissociate what she does from what you do and go on from there
What I mean is, think of her as having a disability, which she clearly has. Like a PP mentioned, self esteem issues, attachment issues, a love junkie, who knows. Asking her to do what you want her to do is like asking someone who has lost their legs to run.....they're just not capable of doing that. You cannot be angry at them for not doing what you want them to do, they just can't. Emotional sickness is similar, we just can't see it, and your sister as you describe her is emotionally very very sick
Now comes the question of, if she is unable to thank you, do you still need to be thanked? You are hosting something to be nice, to keep the family together, to keep some contact with your only sibling, just like you would want your kids to stay in touch with each other. So is it just that she's not doing what is the right thing to do (which she can't, remember she's disabled) or is it that you need acknowledgement regardless? Because if it's the latter, and if your parents are not acknowledging what a fantastic daughter you are, then treat yourself to something really special (dinner, a massage) after you are host the family meal and tell yourself that is your reward and thank you to you for being kind, for being selfless, for being a great example to your kids and for doing something that holds the family together.