A 20 year old boy in the states is becoming successful as a home-visit haircutter and has no real training. He has the opportunity to take an online course today (in the three weeks but before Rosh Chodesh) which will improve his skills professionally. He is working to make his own money because his father is out of a job due to Covid-19. He said that he will not have time to do the course after Tisha B’av because he will be working then. In order to do the course he will need to practice on someone’s hair. His younger brother (15 yrs old) is the only one he could realistically use for this . Is this allowed?
If you have Sefardi minhagim, then it would be permitted, however if you have Ashkenazi minhagim, you should not practice on your brother because he is no allowed to get a haircut during this period. If it is possible try taking the course, take good notes, etc. and you will be able to practice on him and many others in another week and a half. In the zechus of keeping the halachos, you should have an abundance of parnassa.
Rema 551-4, Poskim