My Sexy Sister

I am a normal male, 22 years old and without a girlfriend at the moment. Although, to be honest I have never actually had a girlfriend as I am sports mad and spend a lot of time actively taking part in physical activity.
I live at home still with my parents and sister, (Maria) who is about 18 months older than me at 24 years of age. She has a steady boyfriend and has been going out with him since about November 2017. Our parents are into singing in a community choir and they are both involved with other organisations so they go out several evenings every week. Maria and I started playing board games on a Tuesday evening many years ago when the parents went out. Sometimes it might be chess or Monopoly or any other game we fancied.
Maria and I get on exceptionally well and are probably best friends as well as brother and sister. We find it easy to share secrets and confide in each other knowing the other will be confidential and respectful. She is not beautiful enough to be a calendar girl but is quite good looking. She has red hair and pale skin. She is the type I could fall for and indeed would be happy to date her if she wasn’t my sister.
Quite often, I would sit in an armchair with a coffee table in front of me and Maria would sit opposite me on the floor whilst we played the game. Quite often she would have a skirt on and she didn’t worry too much how she sat. She was happy to sit cross legged on the floor and I regularly saw up her skirt and saw her knickers under her tights and always saw lots of leg.
I was inexperienced when it came to girls and I knew what was under her knickers but never really thought about what she did and just accepted it was probably normal behaviour in her own home. In fact, now I think about it, she spent more time sitting on the floor rather than in an armchair anyhow.
Now I have set the scene, I want to go back to April 2018. This made me to be 20 and Maria was almost 22.
Maria had been going out with Steve for about 5 or 6 months and we still maintained playing games on a Tuesday evening but now with Steve joining in. Maria continued to sit on the floor, sometimes facing me and other times facing Steve. She still sat cross legged revealing up her skirt and generally displaying lots of leg.
One particular evening, we had played a game and Maria revealed her legs as was normal and sat cross legged. Once the game was over, Maria went to get some drinks and Steve went with her to the kitchen
I heard them chatting but only picked up on a few snips of their conversation. The bits I heard were quite interesting though.
“It’s not right to sit like that in front of your brother” I heard him say
“Always done it” came the reply. “I don’t see any harm in relaxing and being at ease at home and you’re the one who wants me to flash my legs when we’re out”
“That’s different. Its to other guys, not your brother”
“And you’re the one who wants me to leave my knickers off when we go out and I wear tights”
“Yeah, but I bet you wouldn’t sit like that at home if you had no knickers on”
The next Tuesday, Steve rocked up about 7.30 and Maria suggested we all play a game. (Maria was wearing a fairly short skirt by the way.) I sat in the armchair with Steve opposite in another chair. Maria sat on the floor next to Steve and facing me. After about 15 minutes Maria changed position slightly and pulled her knees up. I saw up her skirt and something didn’t seem quite as normal but I couldn’t decide what. About 5 minutes after that, Maria moved again and was now sat with her legs crossed. She asked me something as she crossed her legs and I was looking at her.
I saw right up her skirt and saw her tights which didn’t have a reinforced gusset as I know some do. And I saw her vertical smile. She had no knickers on and I was looking at her pussy clearly visible through the nylon. I had an instant hard on as this was the first live fanny I had ever seen. It was a beautiful sight. She stayed like that for probably half an hour or so and though Steve couldn’t see what I could see, I had plenty of views of her gorgeous pussy through her tights. Pubes and flesh were clearly visible to me.
We had drinks and sat chatting for a while. Maria was now on a settee next to Steve and opposite me. She still continued to reveal her fanny to me. My hard on was constant and I could feel pre cum oozing out. A while later, my hard on has subsided so I made my excuses and went to bed. I wasn’t particularly tired but needed to picture my sister again whilst having a wank. And what a brilliant wank it was.
Next evening, I braved asking Maria about what had happened last night and why she didn’t have any knickers on and revealing all to me. As I expected, she was open and honest with her answer as she knew I would keep it a secret between us.
She explained that after four or five dates with Steve, it was obvious he wanted to fuck her. She said she wasn’t ready but would do other things instead. Steve had said that he wanted her to wear tights but without knickers when they went out with his mates on a Friday evening and she agreed to do it. She went on to say that on a few occasions she had left home with a skirt on and a mini skirt underneath and had removed the top skirt once they left home. She knew that a few of Steve’s mates had seen up her skirt and they realised she had no knickers on.
She then mentioned that the previous Tuesday, Steve had questioned her sitting cross legged allowing me to see up her skirt. He couldn’t decide if he found it sexy or disgusting. He certainly thought it was wrong for me to do it. He then dared Maria to sit cross legged without knickers for me to see and she had taken up the dare.
She apologised for doing it and hoped I didn’t get offended or upset in any way.
I assured her that I wasn’t offended, just surprised and initially a little embarrassed. If that is what she wanted to do, that was entirely her decision. Secretly, I wanted more and as she had been open and explained things, I told her that I would be happy to see it happen again.
It all leaves unanswered questions.
Is it wrong for her to do it?
Is it wrong that Steve obviously likes her revealing herself to me?
Is it wrong that I like it and always get hard and quite simply, I now want to fuck her?