How would i react?

My dad is having an affair and he knows that we knew all along..well, he never really brought the topic, he's always out of the country b-because of his job, but now he's here and I just found out the truth about my half brother, well, I kinda had the...

idea about him, it doesnt bother me at first since I havent met him yet.. Now dad forgot to remove the picture from the manual and now I have it..and I think he doesnt know that I have it..what shall I do? Should I tell him that I have the picture and ask about him?

I know its his kid, he looks like my sis, he's like 6-7 years old..

I read from your letter that your Dad is a good Dad with you, and possibly with the other kid too. So if your relationship is good, then you may bring up this question and tell him that you are ready to meet your half-brother. Note his reaction. If he holds aloof then do not touch this question again for few years. Later, when you will be adult you may find the contact to your half brother. Even you may send your half brother some personal presents, like a drawing, or something like that.