My older brother refuses to fast on the Fast of the First Born. Since becoming observant, I have been fasting on his behalf (I am the second born). Is there any merit in fasting on his behalf? I realise that I am not obligated to fast in his place, but thought that it might be similar to contributing to tzedekkah on his behalf.

You can make a Siyum and your brother will participate and will not have to fast. However, you cannot fast for your brother. Rabbi Yitzchak Grinblat