I think there are some great comments from Elena and Military Mom. I also questioned why you felt your 20 year old requires gifts. I understand the wide spectrum of autism and I have many friends with kids who are not neurotypical. But they are adults. If your sister isn't buying gifts for adults, why is it upsetting you so very much? You're equating gift-buying with how she values you, and them. There could be a million reasons - finances, not knowing them well enough to choose gifts, wanting to lessen her holiday load, wanting to lessen your load of having to reciprocate, or possibly feeling under-appreciated.
I stopped giving to certain relatives after years of them never coming to anything (so we could continue to know/have relationships with the kids) and never, ever even writing a thank you note, let alone reciprocating. We stopped giving to others when they just spent a ton of money and bought, say, 5 copies of a best-seller and 4 pillows from some charity they knew we would find offensive, and more of the same. They piled everything on the dining room table and then started putting tags on them. So my mother and I (who spent a lot of time together) always opened the same gift. Or, my mother got the same book as my husband did. This went on for years. It was totally stupid and insulting because there was no thought to it at all.
On texting: I don't find that particularly comforting. I have friends who send out group texts all the time and all I hear after that is the "ding" of replies as everyone on the group text (half of whom I don't know) reply, "Thanks. Same to you." Ding. Ding. Ding. There's another relative (a stepdaughter) who never, ever comes here for Thanksgiving despite it being her father's favorite holiday. Then she makes a big deal out of saying she's so sorry she forgot to call or text - when in fact that's just a reminder that she's not here. So a text during Thanksgiving dinner really says, "Oh, I'm too busy with people who are more important than you are, but here's a text." I used to bust my butt for years to be the perfect stepmother, and finally I realized it was as much about my need to be liked and admired than it was about doing what they really wanted.
For birthdays, it's just a way for people to cover that they didn't buy a card in time. And I can't imagine people adding to their Christmas tasks or interrupting their festivities by making a list of all the people they need to text.
So, instead of getting all worked up over this, why not decide what you want to do regarding your sister and her family. Will it make you happy to give her something? Or do you feel it's your obligation in order to get a gift in return for your kids? Do you have other issues with her and you feel she's not a contributing member of the family? Do you feel she does stuff with her in-laws and you feel rejected?
If you still feel you need to give something to her or her family, why not do a family gift like a package of food (homemade or store-bought) or something from your area? Or, give a new Christmas ornament or decor item. Send preserves from a local farm or something along those lines. Scale back and see if maybe it actually frees you up and makes your life more relaxing.