I have unlimited calling but limited texts - far more common than the reverse.
That's just not true. It may be true for the people that you
know, but it is not a universal truth. It is actually absolutely inaccurate by any measure of industry data.
This thread is reminding me of the TV show Community where everyone laughs and teases one character because, "Britta still uses her phone LIKE A PHONE!"
I think it's a fairly immature but probably too common perspective that a cell phone is not meant for making calls. It's a phone. If you can't/don't want to answer, don't. Not so hard!
I wouldn't call it immature. But it is very common. And it is a sentiment that is growing quite quickly across North America and the world. I don't think it's very nice or polite to suggest that a growing segment of the population is immature. I'm sure you wouldn't like it if that growing segment suggested that you were too set in your ways to see the realities of the world today. (I'm not suggesting that. Just saying that you shouldn't call a whole growing trend "immature" just because you don't agree with it.)
I understand that there are people out there that think that mobile technology is the down turn of civilization and everyone was much better off before we "had our phones glued to our limbs", or whatnot. But, the fact is that it's not going anywhere and to suggest that etiquette doesn't have to evolve with new technologies, is naive.
I'm not saying that everyone has to text everyone before calling them. In fact, I don't do it myself. But, if it's the norm in your group (which it clearly is with the OP's sister) then you should follow the norm. And, if you know it's someone's preference and you have the means to do so, you should follow that preference. It's just being considerate of their time.