I don't know how this happened!

I don’t know how it happened!
I’ve just got through a messy divorce and haven’t really been in the best frame of mind since. I’m the oldest of three children (46), I have a brother (43) and sister (40), both of whom I looked after when we were young and our parents divorced! This is a true story, but to protect our identities I’ve changed our names. Late one night a few weeks ago my sister rang me in tears and told me that she had kicked her husband out and was demanding a divorce, she just couldn’t stand him having any more affairs! I invited her over to my house after school run the next day and she told me through sobs that she’d be here by about 9.30 the next morning.
Beth arrived on time and burst into tears as soon as she got through my front door, I put a big brotherly arm around her and lead her into my living room, sat her down and cuddled her face into my shoulder and just let her cry and ramble through her emotions! After about twenty minutes of this she started to calm down and then went very quiet, she lifted her head from my shoulders and looked into my eyes. The next thing I knew she was kissing me hard on the mouth, forcing her tongue into my mouth, and to my shock and horror I started to passionately kiss her back, one thing lead to another and soon I was squeezing one of her breasts through her clothing, and we kissed harder. Then she pulled my belt undone and had my jeans open and her hands were pulling my rapidly stiffening cock from my underwear, Beth broke away from our kissing long enough to gasp out;
“Fuck me, fuck me hard!”
and with that she both pulled her skirt up around her waist, grabbed my hard cock in one hand pulling it towards her pussy, whilst pulling me down on top of her with her other hand. Once laying there on my sofa, she reached down with her other hand and pulled her knickers to one side before pushing my cock into her very slippery pussy! I don’t know why but I couldn’t stop even though I knew what we were doing was wrong and we fucked like that for about twenty minutes or so before she started to cum, and the feeling of my own younger sisters pussy muscles squeezing my cock as she came took me over the brink and I started to cum, squirt after squirt of sticky cum deep into her belly! That was the most intense climax that I have ever had and it felt like I was cumming gallons! As the last jerks of my hips died away Beth clamped her legs around my waist and held me to her and for a few minutes we both cried.
Beth lay there stroking the back of my head and quietly whispered to me;
“Dave, remember when we were young and you used to take me to primary school?”
“Yes”
I replied,
“Do you remember the time when I was nine and we did a demonstration of Jiving and we did all the jumps and lifts and things?”
“Yes”
“Do you remember the jump where I used to land on your hips with my legs either side of you and you used both a thrust of your hips and your arms to move me to the next position?”
“Yes”
“That was the first time that I felt the hardness of your cock against my knickers and pussy and it made me feel all strange down there! From then on I wanted to do naughty things with you. When you used to have girls back to our house I used to spy on you through your key hole, watching the things that you did with them and imagined that it was me that you were doing them to!”
I didn’t say anything, and Beth went on to say that every man that has ever fucked her she has always imagined that it was me that was fucking her! At that moment my cock chose to soften to the point where it slipped from her pussy and she somehow rolled us onto our sides and she slipped off the sofa onto her knees and before I could put my cock away had it in her hand and then had it in her warm mouth sucking and licking it clean before putting it back in my underwear. I was still shocked at both what we had just done and what she had just said and just lay there not really knowing what to do! It was Beth that knew what she wanted, and it was her that snapped me out of it by taking hold of my hand and saying to me;
“Come on Dave, come and make love to me again, make love to me the way that I saw you make love to all those girls when we were young.”
I did try to tell her it was wrong, but my arguments didn’t even sound genuine to me, let alone her and she lead me up to my bedroom, we stripped off our clothes in silence, Beth was the first to lay on the bed. From where I was standing I could see her bald pussy lips and tops of her thighs covered in the mixture of our first passionate fuck, and as she allowed her legs to drop wide open I saw more cum starting to ooze from her opening! Moments later with my cock starting to stiffen again I was lying next to her on the bed kissing her again!
I broke away from kissing her mouth and started to kiss my way down her throat and she moaned, the sound that only means one thing! As I reached her shoulder she breathlessly gasped out for me to stop and she gently pushed me away from her saying;
“Wait, just watch me do something first.”
Leaning on one elbow next to her I watched as she spread her legs open wide and reached down with three fingers and scooped up loads of our mixed cum that had leaked already leaked from her pussy and covered one of her nipples in it, then she reached down and this time pushed her fingers into herself, again covering them in our cum and then covered the other nipple in it! With one hand she took hold of one breast and with the other she pulled my head to it and gasped out;
“Suck my tit clean darling suck our cum off it!”
Which I did willingly, and when I’d finished that one she moved me to the next, all the time squirming and moaning loudly. It was as I was sucking on the second nipple that I reached down and started stroking her clit, rubbing the mixed cum into her pussy lips and easing two fingers into her curling them until I found her G spot and started stroking it! In under a minute I started her climaxing, and I kept her climaxing several times in a row without a break until she begged me to stop!
Still gasping and breathing heavily she pushed me down onto my back and started to suck and lick my pre-cum dripping cock, before laying on her side, guiding my cock into her very slippery pussy from behind and moving herself up and down on it until it was covered in our slippery cum again, then she stopped and whispered over her shoulder;
“Fuck my other hole, darling brother!”
As she pulled her bum cheeks apart! It took a few attempts at getting the swollen end of my cock up her arse, but as soon as the swollen end penetrated her she started fucking herself hard onto it forcing it quickly and deeply into her, we fucked like that for several minutes and then I rolled us both over so she was on her tummy and holding her hands down onto the bed I fucked her really hard causing her to climax twice, but it may have been three times before I came deeply in her!
After we showered together, she left to pick up her kids from school, with a promise to return another day for more naughtiness! I have tried to write this the way it felt…… Breathlessly, therefore the punctuation may seem a little off, I apologise for that!