"Pray for my brother!"
Translation:התפללו בשביל אחי!
To pray for someone, you would use התפלל בשביל or התפלל עבור. On the other hand, התפלל ל or התפלל אל means to pray to someone/something.
So, your sentence means "pray TO my brother". So, you can say that, but it definitely means something completely different than the sentence provided for us.