First, I wouldn't recommend bringing up the other employee's pay increase, unless for some reason, the lack of equity could be evidence of illegal discrimination. Prepare a case for the salary you want based upon market data and upon the clear increase in your workload. Just in case an increase is impossible, you should have a fall-back position such as a substantial bonus and plan to shift some duties to other employees, a flexible schedule, etc. Then, rehearse your pitch with a friend or partner to come up with some good phrases and delivery.