The word תִּמְשָׁל is a second person future imperative of “to govern”, so it quite literally is an order “you will govern”. It doesn’t translate as “do thou,” “thou shalt,” or “thou mayest.”
Timshel: thou mayest.
“Thou mayest rule over sin.”
Steinbeck claims that this is important because, since Cain slew Abel, we are all descended from Cain. God’s promise is then that we as humans have a choice. If we choose, we may rule over sin.