May I use Maaser money to help a relative who is a Shlucha, to pay for flights and other needs on their upcoming Bar Mitzvah?

The flight may not be paid for with Maaser.

If the Shluchim qualify for Maaser, i.e. they cannot make ends meet, you may buy for them their needs from Maaser money.

If you are buying items for the Chabad House or for their outreach activities it may be bought from Maaser.