You’re being ridiculous.
Not least for the missing out on a BOGOF, which you didn’t.
Your family has a tradition of providing gift lifts for “things” not vouchers. Fine.
This year, you know your sister intends to give experiences.
So why are you grumbling about her asking for “things” when she hasn’t even done that yet?
Just message her: “good idea on experiences not things, happy with that. When will you be sending a list of what experiences your family would like? I’ve had a think, and we’d love Cineworld* vouchers, please.”
*that’s me picking something that most families can use, regardless of SEN - or at least are great for re-gifting. Obviously, you decide what’s good for you family
Stop being so passive, and complaining about something that (a) hasn’t happened and (b) you can stop
Even if she says, “oh no I want you to get me things” - what’s the big deal? It’s all one massive transaction where you just shop off a list anyway. Which is either very practical and avoids waste, or is utterly joyless - depending on your personal point of view. But whichever you think it is, if you’re providing a list then it works.