AIW for brushing my wife’s hair while we’re watching a movie with two other couples?

So a few weeks ago we went out of town and shared a rental with two of my siblings and their families. One evening we adults were watching a movie in the living room, and my wife came back from putting our daughter to bed holding her hairbrush.

She handed me the brush, sat on the edge of the sofa between my knees, and I brushed her hair. There was nothing sexy or weird about it; we were both still watching the movie.

But my sister says this is inappropriate contact in the presence of others! I told her she should just face the screen instead of us if she doesn’t like it; but she complained it was like watching us engage in foreplay!

Now my main question is just about combing my wife’s hair in front of others in this setting. But, admittedly, when she said ‘foreplay’ I had to say the next obvious thing about her maybe learning something from watching us. At that point everyone started chuckling which pissed my sister off. The whole group tends to the crass side that way.

For background, I brush her hair almost every night, because I like it long and she threatens to cut it short if I don’t. I’ll also add that my sister is two years older, and lots of people have noticed she’s often a bitch to me.

My sister still won’t let this drop and says she won’t travel with us again. Personally I think going forward we’ll just rent our own place.

EDIT: fixed a typo, added some more context and tried to answer questions. Thanks to everybody who opined on the hair combing itself, I appreciate it!

More context: I'm 28 and she's 26, we met four years ago and have been married for two years (there is no 'weird' age difference); we live in a condo in a big city (not in a rural compound); we both work full-time (actually met at work and, no, she did not work 'for' me in any capacity; our bosses knew when we started dated, small office; currenty we work for different firms).

I am not dictating her hair length! Reread that paragraph, this time put quotes around 'threaten' to make the dynamic more clear? When we met her hair was to her waist. When we were dating she cut it super-short, which I also like; then she grew it shoulder length and said that I'm 'required' to comb it. Hope that helps. This may also help: my wife is the sole owner of the condo we live in; otherwise we pool all finances (including the mortgage). So our primary residence is her pre-marital asset.

My sister deserves her own post! Here's the issue: my sister seeks control of me as personal validation. It is absolutely a bad relationship, and I thank one of my best guy-friends, an ex-college-gf, and my wife, for helping me navigate away from her to my own space. The ONLY reason we see my sister at all is for the sake of the cousins knowing each other; we would never/ever leave our daughter with my sister.

The dynamic of this couples group (us, my two siblings, their families) is super casual. For example, we also travel with some of our work-friends and their families and in that setting I would NOT comb her hair in front of the group. Multiple people commented that the hair combing is like trimming nails; I beg to differ!, but point well taken. Also hilarious because I hate it when people clip their nails at work (which, I think we can all agree, is wrong?).

Ultimately, the problem here is that the one person complaining (my sister) is also the one person that I have deep interpersonal issues with; fortunately, that problem is easily solved, because we are never sharing a rental with her again. Thanks again for everybody's attention.