Mother is dating man with two girls who are competeing for attention with my sister and i?

Ack, gag, hairball!!! You really want to get into a pissing contest with your mothers boyfriend's kids? You know who loses that contest? Your Mom that is who. Of course they don't want to share their Dad. Would you if he was your Dad? You are a jealous snot and you will only make trouble bigger than you already think it is if you set out to one up them. This man may be your mother's ticket to a comfortable old age so just suck it up. At 17 you will be out of the loop in only a year or two, the two younger girls might be able to get over themselves and become friends if you keep your spite to yourself.