About 3 and a half weeks ago, we bought what I believe is a juniper bonsai from Longwood Gardens. The instructions received with the tree were to water every day. The tree is in mostly sandy soil (very little soil at all that I can tell) and has been kept indoors in a room that gets sun from the south. I noticed yesterday that a distal branch is starting to dry out. If I dig into the soil, it feels moist but not wet. Again, it is mostly sand, so it is hard to tell. I have a Rapitest hygrometer that indicates it is ~2 (dry). Right now, I cannot tell if it is getting too much water, too little water, or needs to be placed in the cold (some sites I have seen suggest this is an outdoor plant). I do not suppose that trimming the dying branch will be of any use. Any help would be much appreciated.

Moonrabbit wrote: has been kept indoors in a room that gets sun from the south.

Junipers do not belong indoors.
Junipers should not be watered every day, they should be watered when needed.

What you have purchased is a young plant and currently not a bonsai.

You can cut that branch, it's dead (at least, the brittle top part is). As it seems the only one affected, it may just be mechanical damange and not much to worry about (yet).

they need sun or they may turn brown remember to water when soil gets dry

Rag wrote: they need sun or they may turn brown remember to water when soil gets dry

This damage is not caused by lack of sun

I have been following everyone's recommendations, but the tree does not look like it is doing well. I have been keeping it outdoors and watering it only when it appears to be dry, typically twice per week. I have a toothpick in the soil that I use to gauge dampness. The plant has lost some of its color. The leaves are largely still soft to touch, but in some areas some of the needles on the ends of branches have turned brown or stiff/crumbly. The sickened branch I mentioned earlier (not shown in attached picture) has gotten worse, and I am probably going to have to cut it back to the trunk.

what is the weather where you are? Do you get cold winters? If you get frosty winters junipers turn colour as they get more frost hardy. Do not trim till the new growing season starts, unless you are styling. But not because you think branches are dying.