Is Zakat obligatory on money given to a brother or sister as a gift or a loan? And is it permissible to continue fasting with the intention of marriage?

The money you gave to your two brothers is a gift (hiba) that is perfected by possession and acceptance. If they did not accept it, it returns to you as your property. Since you do not oblige them to guarantee it against damage or loss, if it returns to you, you are to pay zakat (definition) on it for one year, even if it remained with them for a long time. As for fasting perpetually (siyam ad-dahr), if you mean fasting on days forbidden for fasting, such as Eid and the days of Tashreeq, then this is not permissible. But if you break your fast on the forbidden days, then it is permissible.