In my graduate application, should I briefly mention a family issue? Last summer I wasn't able to get much research done due to a complicated issue in my family which cost me a lot of time and emotional attention. Since it was not simply that I was sad and lazy for an entire summer, but that I was actively helping with an urgent issue in my family, I feel like it may help to briefly - in a sentence or two - explain what caused the setback and why I have grown from the experience. However, I don't know how harshly admissions committees will critique this "excuse". 

Is it generally better to write about it, or to ignore it altogether? I'm asking this because I don't want to seem like I'm trying to hide a potential setback in my application, but I also don't want to shoot my own foot by rescuing what was never in danger in the first place.

I am applying for physics PhD programs in the US.
I don't think it is relevant to your applications.  It was a short period, and many students take advantage of the summer break for such things as travel, time with family, gainful employment, volunteer activities.

It would be different if you had had a break from your studies of a year or more.

Best of luck to your family.