A word for something that's done only half-consciously

In writing fiction, I find myself using the word "absentmindedly" a lot, but I don't think it's really the word I'm looking for. I'm not looking so much something done in a distracted manner, but something done sort of half-consciously, without really thinking about it. Not habitual either, but perhaps almost instinctual.

For instance:

As James watched the other girl climb higher, he shuddered to think of his own sister being in such danger, and put his arm absentmindedly around her.

What I'm trying to relate is that he wasn't making a conscious move to comfort or embrace his sister, he just did it without thinking.

The word instinctively fits here.

As James watched the other girl climb higher, he shuddered to think of his own sister being in such danger, and instinctively put his arm around her.

The action is done without thinking about it, as an innate reaction to thinking of the danger and protecting his sister.

My initial thought is sub-consciously would be a suitable choice.

I prefer that to almost un-consciously suggested in some of the comments.