I would say para. Because para means "to" and the sentence is saying "my brother bought a bike to ride in the mountains"
Los 3 cochinitos es un buen cuento
1) Juan and Sofia doubt that
2) lili and don jose are not likely
3) the sports club denies that
4) it is not possible that antonio and mauricio
5) Doña Lucia does not believe that
6) it is impossible that pedrito and you take
7) Felipe and Sister Piedad are not sure that
8) it is impossible that Guillermina and Jorge
9) it is not true that lili and sofia
10) it is not true that the sports club
11) it is not certain that don josé and juan
12) doña lucia and guillermina do not believe that
