My son's behavior scares me

I have discovered a few months ago that my 11 years old son has played "pretend sex" with his 6 years old sister. After long conversations a many tears we have decided to try to make this go away by assuming this was not his intention to hurt or harm his sister in any way, he said he knew it was wrong but he didn't realize it was "that" wrong. our daughter who is now 7 years old was playing some computer games when we came in the living room she suddenly closed the computer obviously scared about us finding what she was watching...we found out she had tiped in youtube and after that "nakid girls" so she was actually watching some teen girls kissing each other.
We are overwhelmed and really don't know what to do...I am so sad and so mad at myself and at my son who was always extremely smart and feeling that I have failed to protect my daughter and her childhood.
I really don't know what to do, how to react...what is the right next step...my daughter told us her brother had showed her this website and what to tipe...he had watched it a few times with her. This is very hard for me and I really would like to know if this is a society problem or is it just me who failed in protecting my children.

[margypops]

Perhaps a good idea is to supervise the use of the computer, be around when she has it on, there is a lot of problems with parents allowing children to have full use of the computer then wondering why when these things happen , so my main suggestion is less of it and be there when she has it on.