Doesn't my opinion matter?
Of course your opinion matters. It's just not the deciding factor though. You haven't said WHY you don't want him to get married, only that the person he fell in love with is only five years older than you.
Have you noticed how he's changed since he met her? Does he seem more happy, willing to do things he hasn't done in awhile or do new things? Does he smile a lot more, act with more patience or easier to get along with?
How's your relationship with her? Do you think that she'll take time away from you or try to act like a mother to you? Have you had a chance just to spend time with her alone, just to see who she is?
How's your relationship with your step father? Do you get along or just stay away and do your own thing?
You have to understand your dad doesn't want to make you UNhappy, he just wants happiness for himself. Really think about making him choose between his children and someone he loves enough to marry after all this time.