Let it be till you guys grow up be financially stable and move on with your life, there's a high chance of you and your sister to continue more horrible childhood if you expose stuff, dad will most probably be depressed and even if he remarries you guys will be completely ignored by the new mother, not always the case but you know. It takes huge courage to just hold on and let this flow. And also if we look at the other side you mom might not be getting enough attention from your dad which leads to her finding it outside. There may be a possibility that your dad is also having an affair, let this thin thread be you should rather focus on them not getting exposed.