Fuck me, a whole day as a trainer, and a whole hour of training given to the piercers? And that's considered good by Claires standards.
A proper piercer, who is very unlikely to be a hairy tattooed biker (although I am a female, long haired, single tattooed, multiple pierced, biker myself) will have done a minimum 6 months apprenticeship - more likely upwards of a year.
In that time, they'll have had to learn about hygiene, sterilisation processes, autoclaves, first aid, blood borne pathogens etc.
They will have had their Hepatitis inoculations amongst other things to protect themselves.
They will not try to sell you bullshit magic potions to heal the piercing more quickly as there is no such thing. They will offer free aftercare. They will want your piercing to look it's best as it's their reputation on the line.
They will understand the need for different lengths of jewellery to allow for swelling - something the guns can't do. My ear lobe is actually too thick for the standard Caflon studs, and they embedded in the back. This should have been checked by a competent piercer and an alternative offered, but was not as it was done in H Samuel back in the 90s with a gun.
Piercing studios are licensed by the council and cleanliness standards stringently maintained. Their piercing methods are quicker, quieter, less painful, more professional, fine by far better trained people, worth better quality jewellery for less money. Why people insist on going to Claires is beyond me.
However back to the OP, your SIL is an arse and needs reining in.