Your mom is right about it being a sin. In the bible, it says, “You shall not make any cuttings in your flesh for the dead, nor tattoo any marks on you: I am the Lord,” Leviticus 19:28.
Though, I can understand the want for them. I got one a couple of months ago in my daith.
I think the biggest thing with your mom is making her understand that you will take care of it properly (so no infection occurs) and that your experience won't be the same as your sisters'.
Since your mum is scared of something going wrong, maybe a belly button piercing would be best? They are one of the easiest to maintain and it's rare they get infected.
And, hey, the worst thing that would happen is you have to wait a year to get it.