Hi when you sister joint a broken beam with the same size wood and 4 ft in each direction from the crack and 3 nails every 12 inches does the beam become stronger? the same as it was before the original beam cracked? or weaker than if I would replace the whole beam
If the crack is so severe as to render the original beam essentially failed, then partial sistering (which is what you describe) will restore a portion of the original strength of the beam prior to cracking. If the crack allows the beam to retain some strength, then it would require a careful detailed engineering analysis to answer your question.