I'll offer another perspective (I don't know that I agree with it, but I understood when my mom explained it to me). She finds that it is contradictory to be in the communal area yet put off solitary vibes to the people who are there. Putting in the headphones to her shuts out everyone else, which is fine if you are in public with strangers who have no expectation of communication or if you are in a separate room for alone time. However, if you come and are sitting in the living room where people in the house gather, she feels bad when you then don't "gather" with the others who are there.
It's like eating dinner as a family or at a dinner party - there's a social expectation that people will participate in conversation and carry their weight. If you joined the dinner table and then expected to just eat your food while not interacting, it would put others off for sure.
There's also thE fact that you cannot share what you're doing with other people. They can't hear your music, or watch your TV show with you. Both of those things would help them learn more about you, create shared experience (which bring people together).
It's a very isolating thing. And inside the family, that's really awkward. And it's extra awkward if you're doing it in the shared space.
The same can be said for reading a book, writing, solving a puzzle, drawing, etc. There is a time for social interaction but there is also a time for more individual activities. Doing them in a shared space (as long as it is not disturbing) is a wy to make it less individual, not more.
Yes! I've had some lovely mellow evenings where my SO and I both sat and read or wrote, while snuggling on the couch and making occasional idle conversation. I really value that in a relationship, actually--the ability to sometimes do our own thing separately-but-together. Obviously this isn't every interaction, but sometimes.
Note that you can make the occasional idle remark. The trouble, for me, with ear buds or bluetooth, is that you really can't. In fact, as the OP says, you have to attract the other person's attention first, then talk. Not something I'd want to do every time I had something unimportant to say.
For me, the use of earbuds at home, as opposed to at work or on the subway, etc., signals that the person doesn't want to be interrupted. (I almost typed "bothered by other people," because that's how it seems to me.) It's one thing to send that signal at work, another thing completely to send that signal at home.
And it's to the point with the OP that she simply doesn't talk to her SIL much of the time, because of the need to check to see if he's occupied with a conversation or music or whatever he's listening to.
And that's a problem for people who share housing. The OP should be able to say, "Dinner will be at 6:30 tonight instead of 6:00," or "Please pick up some orange juice on the way home from work if you want some tomorrow," without having to spend time checking for earbuds, making eye contact, waving her hands in front of SIL's face. Getting his attention could take longer than delivering the short message she needs to give him.
Because the habit of using bluetooth or earbuds is affecting the other members of the household, i.e. the OP, I think maybe having a talk about limiting the use of the devices, or restricting which rooms they can be used in, or some other compromise, might be in order.