I don't know how to deal with this or bring it up?

My dad has been married to my step-mom now for 6 years. Her and I have never really seen eye to eye and to be honest with you I think she wishes I didnt exist. My step-sister got married 2 years ago and they put about $7,000 into her wedding and now I can't even get my dad to buy me a lousy $250 wedding dress. My step-mom was talking to a good friend of mine(idiot) and told her that "they don't have the money to pay for a wedding" Of course she came back and told me. But what I don't get is my dad doesn't have the money to pay for my wedding but he can go out and spend $300 a month of medicine for a dog and just last week they went out and bought a $200 puppy. I'm not trying to be bratty but do you not think its fair that they won't even buy me a wedding gown?!?! I am his only biological daughter. Idk I just don't think they are fair to me I have worked since I was 14, I alway had to buy my own stuff and I was paying rent at age 16 while my step-sister, her husband, and son lived there free. I know there is that responsibility level but don't you think it is a bit much?! I dropped out in the tenth grade when I was in all honors cles because they never cared that I was ranked 2nd in my cl or that I won 1st or 2nd place in science fair and FFA events. But when I had money they were my best friend. As of right now they owe me $700 from my savings bonds which I cashed to pay their bills for them when I was 16. The only time they call me is when they want something, I had to call them on my birthday. My dad won't even talk to me about paying for the wedding...he told me that I should rent my wedding dress...well jeezzz DAD why can't you spend $250 on a dress you spent $600 on a dress for your step-daughter. Would it be wrong if I told him I didn't even want him to walk me down the aisle. I don't even know if I want him at wedding...I feel like he just doesnt even care about me anymore. And it breaks my heart so much because before my stepmom I was always a daddy's girl....I can't even call him my dad anymore because my stepmom yells at me for being selfish...she says hes not just your father he has stepchildren too. What's your opinion?