Translation:My sisters won the first prize.
I think it is worth highlighting the difference between the portuguese language perspective and the english language one. In english, we would not include the word "the" — it would just be "my sisters won first prize" — meaning, they took top honors. including "the" turns it into a comment about the sequence of the prizes — often they are actually given in reverse order, culminating in the top honor... so "won the first prize" almost has the reverse connotation.
In portuguese language, if Paulenrique's response (and the question's rejection of "my sisters won first prize") is correct, the two are intimately bound together. The fact that it is sequentially first apparently means that it is also top honors.