How can i earn my step dads acceptance?

i feel like im a complete stranger to him. i try sooo hard to at least be noticed as his son. my bio dad is in prison till im 47 years old so he wont be around any time soon. its just that i have a younger sister (his bio daughter) who he shows every bit of care and concern for her and rarely me (one example is, 3 years ago when she got sick with the bronchitis he took the day off of work to get her to the doctor and a month later when i had a sinus infection i had to wait a few days till the weekend, i just feel so discarded. i used to be confused when i was younger (7 years old) why this was, but now that im older (16 years old) i fell more hurt emotionally than anything (i even cry sometimes) i feel embarred even sharing this with any one. i know im not alone in the world with this problem but i dont know what im doing wrong, i know im not his real son but i dont understand why im so alienated all the time. i guess i just need to get things off my chest once in a while.