Silver solder would be ideal for this joint - I expect it is about 1mm brass.
(Soft solder would be toooooo weak, even with a filler added each side to increase the joint area)
As with all soldering, cleanliness is the whole key to success, (as well as the right flux and enough heat.)
I plan how to hold the parts together for soldering (in this case a loop of copper wire twisted to hole them in the right place) then clean the parts obsessively with file, sandpaper and solvent. dollop with flux, wire up on a firebrick (or brick or soldering mat) heat to dull red and the solder should flow like water. allow solder to go solid then quench in water and clean up mechanically or in pickling acid (any acid or coca cola)
sorry if I am teaching you to suck eggs
My way of avoiding this break is to make the boom/jaws joint as well as possible, pin it with bamboo pins, then cut horizontally with a razor saw right into the jaws and boom as far as possible and glue in a piece of 1/64 ply to strengthen the whole shebang.
I s'pose one could use pre-cured carbon sheet instead of ply for added rigidity.
Another thought that crosses the mind is to cut the jaws complete with a long tang in sheet brass (or printed circuit board), then build up the wood shape around this as a former.