Hi last night i stayed up late and when i went to go to bed i herd my sisters tv was still on so i went to say good night to her but when i opened the door i saw my 12yo sister giving a 23yo guy a blow job what should i do should i take the sick c*nt to the police or what

OBVIOUSLY, the situation should be reported to a trusted adult, or directly to the police. Your sister needs medical treatment and counseling; the guy needs immediate legal intervention. DON'T ask your sister if it's "OK" -- this is a situation in which her opinion does not weigh in because she is much too young to be having ANY kind of sexual activity with anyone. But the sooner this behavior is reported the better, before this guy hurts her or another girl again.
Go tell the police what you saw. That way, it will be confidential. Any other adult than the police do not have the same restrictions placed on them and could cause great shame to your family.
I don't think she's to young to be experimenting but WAY to young to be experimenting with a 23YR OLD!!!!
Take the above real advice and intervene ASAP your sister is being illegally molested!