I think this what was going on. Not that the flight attendant has a family member, but that airlines now give extensive training to flight attendants about child sex trafficking that turn the flight attendants into wannabe FBI agents.

Imagine if the teenager that your father appeared to be preventing from speaking to the flight attendant was an Asian female.

Bottom line, the flight attendant saw circumstances that were unusual to her and gave rise to suspicion. Teenager who looks non-disabled sitting by window with an adult male on aisle is ordering everything for him and preventing him from interacting with the flight crew. If your brother's autism manifests itself in avoiding eye-contact or appearing nervous, this could have given rise to further concern.

Google flight attendant human trafficking and you'll understand more.

Here's a snippet: http://mashable.com/2016/01/12/how-t.../#luq7hLPxgGqf

"A few years ago, my airline started training us on spotting telltale signs of human trafficking on the plane. Can the passenger speak for themselves, or is someone with them controlling what they say? Does the passenger avoid eye contact? Do they appear fearful, anxious, tense, depressed, nervous, submissive? Are they dressed inappropriately, or do they have few possessions — even on a long flight? Can the passenger move independently, or are they accompanied by someone seemingly controlling their every movement?"