How to seal brick and siding corner?
I normally would suggest a water based caulk, but for this since it is brick on one side, and would need to be taped to keep the brick neat, I would use a one part polyurethane caulk for this. The application would have to be done so there is not a whole lot of excess over the area since clean up is done with mineral spirits or paint thinner.
It is available in colors, so you can find something that will blend in. It is also paintable, and is very flexible.
I would go the extra mile and tape the cedar siding too. This will allow you to wipe off all the excess, pushing the caulk well into the gap at the same time, and touch up where needed, cleaning that excess up too. I would use my finger to do so and wipe it clean with a rag or paper towel dampened with thinner. After all the excess is cleaned and smoothed out down to the tape on both sides, remove the tape and spray the joint that is filled with a soapy water solution. This will allow you, with soapy fingers, smooth out any last flaws in the caulk.