Can I keep Dad's second wife from being his executor?

Probably not. If your father names her in his will, then a court will honor his wishes unless there’s a very good reason not to—and your suspicions about your stepmother are unlikely to satisfy that standard. If, after she starts serving as executor, she exhibits signs of negligence or dishonesty, then you would have something to present to a judge.

Meanwhile, you could try suggesting to your dad—without badmouthing his wife—that one of the children might make a better executor. Or, if you think it could be done without actually increasing hostility, that he make his wife and one of you co-executors? If you really think an impartial third party is necessary, your father might consider naming a lawyer or other professional. That could increase the cost of probate considerably, but it’s cheaper than a lawsuit among surviving family members.