In punctuation marks, the apostrophe seems to cause the most trouble for students. The concept in question here is the difference among possession, plurals and the s in third person singular verbs. The correct placement of the apostrophe is between the r and the s as in stepbrother's car. The apostrophe here shows that the car belongs to the stepbrother. The s in needs is there to show that the verb agrees with the third person singular subject; a simpler example of this is "he runs." The s at the end of tires is to show a plural noun meaning that the car would need more than one tire. Plurals do not need apostrophes; therefore, the only apostrophe required in this sentence is the one indicating possession--that the car belongs to the stepbrother. The link below will help you learn how to use apostrophes, and since they are often used incorrectly, this tool could help you a great deal.