NTA. No one is ever ta for not doing stuff for free that they would normally get paid for even if it's family and friends. Especially if it's something that you have to pay to take classes and/or licensing to be able to do.

That being said, it sounds like they are going to expect it so what I would probably do is the next time they bring it up politely but firmly let them know that you aren't going to be able to work for free (but probably phrase it nicer) but you can give them a discount or something (if you can) Honestly they probably are going to be annoyed, but it's easier in the long run to never start doing free hair cuts than it would be to do it free a few times out of guilt and then start charging them or not doing it.