Who Stole My "Parenting For Dummies" Rulebook?!

The girls had a friend over (kaylin)... they were playing in the girls room and they asked me if they could get board games out of the closet and I told them no, after I was done cooking we'd play and to stay out of the closet. They asked me repeatedly and every time I told them no, growing more and more impatient by the minute.. Their friend went home... 30 mins later I'm putting laundry away and walk into their room and two board games are on the floor, a few pieces scattered on the floor. I finished the laundry and turned off the tv and asked them to come to their room. here's the convo:

"girls, please sit down. i'm not happy right now. do you know why mommy isn't happy right now? what's that? (pointing to the board game)"

Girls: board games...

"And where did you get the board games?"

Girls: out of the closet

"And what did mommy tell you about going in the closet?"

KAYLIN DID IT! IT WAS KAYLIN.

"I didn't ask who did it. I don't care who did it. I told you both not to go in there and not to play with the board games. And you did not obey me"

Jayruh: It was Angelina and Kaylin! I'm telling you the truth!
Angelina: Nuh uh! Kaylin did it.

"It doesn't matter. When I ask you two to do something, and you guys don't obey me, you both get spankings. That's just the way it is."

both girls get spankings.

I lay it on Jayruh a little harder than Angelina because Jayruh's been completely un-phased by spankings lately and is her normal giggly self as soon as my hand is off of her. She started crying immediately and I pulled her on my lap and told her that I love her and I spank her because I want her to learn "to do the good things" I pulled Angelina on my lap and kissed her too. And Angelina looked at me dead in my eyes and said "I did it mom. I did it first. Jayruh didn't do it. I’m sorry" Jayruh said very sincerely “sorry I didn’t do anything about it” Talk about feeling like a lousy mom... It’s not Jayruh’s responsibility to make sure Angelina is doing what she’s supposed to. I'm at the point with the girls where if I tell them to do something (like stop fighting) and then they fight, I don't care who started it, they both get in trouble. This way they play nice, and usually they look out for each other whenever I tell them they both are going to be in trouble if either one doesn't listen to me... It’s worked really well in the past (mainly when it comes to fighting or running after each other in the store (something they are both active participants in), but this time I feel cruddy. Jayruh has such a gentle/sensitive spirit and any kind of retribution really, really affects her. When I left the room I got teary eyed... why does parenting have to be so difficult?