Was it legal for a man to marry his brother's widow in 1836 England?

No. Despite biblical examples to the contrary the Christians as early as the 1400s (if not earlier) has very strong feelings about marrying anybody more closely related than a first cousin (and even then it was questionable). You may recall that King Henry VIII of England was so abhorred with the thought of having married his brother's wife Catherine of Aragon that he founded a whole new church to get himself a divorce.