If you are single, you can give $13,000 per year each to as many people as you want including minors.
If you are married, you and your spouse can give $26,000 to each recipient.
If you give more than that in a calendar year, you must file a gift tax return.
Paying their mortgage directly would be considered a gift also. If the property is jointly owned by your brother and his wife, then you could pay up to $26,000 toward their mortgage each year.
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