Ha ha. I am nine years younger than my big sister. We are both ACOAs, too.
It is only because of her better example that I learned anything about polite society, saying thank you, or even realized that our mother was an alcoholic.
But I will say this: the gap in our ages is a lot bigger than it seems at first glance, mainly because of the different experiences that we had growing up. She and our brother (7 years older than me, 2 years younger than her) had a lot of experiences with Mom that I did not: they often had to clean up after her, make excuses for her, put her to bed. I was protected from most of that - by them and by our father. Several years ago it became clear that my sister really resented how "easy" I had it. I wasn't in recovery yet and I lost it on her. How would she know how I had it? By the time I was eleven years old, they had both left me there alone with mom and dad. They may believe they had it tougher, but at least they also had each other.
Not sure of my point. Maybe just that we all have different experiences even though it might not seem like we should. Same house, same parents, why not the same childhoods? Well it did not work that way in our house. I don't know what is up with your little sister or why she can't say thank you. I know that no one ever could stop trying to drink away life or to control each other long enough to teach us that stuff when we were growing up (I don't know how my sister figured it out!). She may not be ready to let you know either or to deal with whatever her own experiences were. My brother sent me a text today to thank me for the Christmas presents. I haven't received anything from him. All I know now is that I am much more peaceful when I let go of all expectations and outcomes when it comes to my family and just focus on accepting that we are all still dealing with things that maybe the others can't understand. At least right now.
Merry Christmas to you and yours, Liz!