Do those who braid a strand of artificial hair for their sister to show how hair extensions are attached fall under the curse of the one who attaches hair extensions and the one for whom they are attached? And what is the way to atone for this mistake?

If artificial hair is made from human hair, then attaching it is forbidden. Teaching a woman how to attach it is participating in sin. Remaining silent about this prohibition is an evil act that requires repentance, remorse, and a firm resolve not to return to it. However, if the artificial hair is made from other materials, such as sheep's wool, scholars have differed regarding the ruling on attaching it. It is more cautious to avoid it and to avoid assisting in it.