I caught my kids self-pleasuring together

I'm a single dad to a 14yo girl and a 13yo boy. For this I'll call them Zoe and lee! Their mother died when they were toddlers and I've been raising them by myself with no major bumps in the road... Until last month!! They have always been best pals. They study, hang out, watch movies ect together. One night I was struggling getting to sleep so I got up to grab a night cap. As I went downstairs the living room door was half open and the light was on. I thought I'd see what movie they were watching (normally horror movies on Netflix.) when I walked in what do I see?! There both naked on seperate sofas watching porn and masturbating! They were enjoying themselves so much they didn't even notice I was stood there. Then I made a mistake.... I said rather loudly "what the f*** are you two doing!". Zoe jumped up and ran out bursting into tears on the way. Lee just froze. I looked at him and knew he was petrified. I told him to get dressed and I'll be back in a min. I went into the kitchen and stood for a minute or so and decided not to go mad at them. I never had any siblings so didn't know if this was normal. Instead I thought I'd try to lighten the mood since the last thing I want so do is upset them even more than they already are. I took 2 beers out the fridge and walked back in to find lee sat with his face in his hands desperately trying not to cry! I sat next to him and nudged his arm. He wouldn't even lift his head. So I said "right you've been caught so I want you to be 100% honest with me....... You got any porn I can borrow!" He looked up immediately and if I'd have taken a picture of his face I'd be able to sell it for a million! I handed him a beer and told him to tell me what the score is. He told me they had been sharing these moments for about a year. It started when the discovered porn movies and they basically got turned on and masturbated in front of eachother for the first time. They have shared everything with eachother their whole lives and they didn't see why this shouldn't be one of them. I had to ask the awkward question which was how far had it progressed? And he said they had never even come close to touching eachother..... Thank god!! I told him that was the line.... And if they crossed it then we have a serious problem. I told him it's normal for kids to masturbate and that I don't mind him sharing the moments with his sister. Then the harder part came.... With lee it wasn't so bad because I've been a 13yo boy before.... But as far as I know I've never been a 14yo girl! I went up to her room and knocked on the door. She didn't answer me but I could hear her crying. I slowly opened the door and what I saw broke my heart. She was standing in the corner holding a picture of her mother to her face saying sorry. As I walked over to give her a hug she quickly turned her body round so her back was to me. I said "it's ok sweetheart" and she just fell to her knees and burst out crying. I picked her up, turned her round and gave her a massive hug. After about 2 mins of crying she said "I'm sorry dad". To which I replied "no I'm sorry for swearing at you". We sat on her bed and if told her that lee spilled the beans on everything. She actually looked relieved. Probably glad she didn't have to tell me everything herself. I told her about the line that's not to be crossed and she looked at me as if to say "ewwww dad that's sick!". So we both sort of chuckled. Then she told me she doesn't have anyone to talk to regarding sex, masturbation, curiosities ect. So I told her I'm always here if she wants to talk. Ideally she would have these conversations with her mother but unfortunatley she doesn't have that option. She'd only spoke about these things with her brother. I told her to come downstairs and she did without hesitation. I sat her next to lee and told them from now on, no more secrets and made them promise to talk to me about these things. They both agreed. We had a little family hug and I said goodnight..... As I walked out I shouted "you can finish now if you want!" They both just laughed. I'd love to know your thoughts and opinions on this. How would you have handled it? Would you go mad? Would you forbid it? Would you allow it? Have you shared these moments with siblings? Call me wrong, call me sick, call me a bad dad! My kids are happy so I'm happy.