Complex sentence: Since Black was unfit to run in the 1935 Canadian election his wife, Martha Louise Black ran in his place as an Independent Conservative and held the seat becoming the second woman elected to the House of Commons (the first being Agnes Macphail).
Split into simpler sentences: Since Black was unfit to run in the 1935 election his wife, Martha Louise Black, ran in his place as an "Independent Conservative". She held the seat, becoming the second woman elected to the House of Commons (the first being Agnes Macphail).