Pregnant Sister

I'm Gary 25 youngest of 5 and work and live on the family farm. Last year in June my only sister Robyn 27 returned to the farm pregnant and out of work due to the pandemic. Her partner Donald had left her and move interstate. Losing her job shortly after that, she decided to return to the farm. Our parents retired to the coast after our father Patrick had a heart attack and left me to run the farm. My older 3 brothers are married and work in the city. So it was just me and Robyn on the farm, Robyn hadn't told anyone else in the family about being pregnant except me. She didn't want to worry our parents and so didn't tell our brothers either. Robyn looked after the house and I ran the farm and all was fine at first. Then I had been cleaning out the pig sty and was hosing myself down out back of the barn and was naked. After hosing myself, I started to hose my clothes hang on a old railing. When Robyn came out of the barn looking for me and caught naked. She just laughed as I tried to cover myself and then told me we needed more firewood in the kitchen. Then she returned to the house, I rung out my shorts and put them on. Grabbing the rest of my clothes headed over to the house and put them in the laundry tub and went inside to get dressed. After I returned to laundry and put my shorts in tub with the other clothes and then got the firewood Robyn wanted. It was almost lunchtime so I waited for lunch in the kitchen with Robyn. She had a big smirk on her face, every time she looked at me. I didn't say anything and during lunch it was Robyn who mention me being naked. She claimed my cock was bigger than Donald's cock even on the slack. Thinking she was waiting for me to say something I stayed silent, she continued by saying she wasn't teasing me and I shouldn't be bashful around her. I nodded and went back to work. At dinner that night Robyn asked me to do her a favor and I said okay what do you want me to do. Wash my back in the shower and I'll wash yours, thinking she was trying to tease me. I replied okay let me know when your ready. Thinking she would back down, she she replied okay and that was it. Later on when everything washed up and put away. Robyn told she was going have her shower and I stayed in my chair, she then said come on you can't wash my back from there. I was still thinking she was teasing me and wasn't about back out of the agreement first. I followed her to the bathroom and entered with her. She completely undressed and told me to hurry up and join her in the shower. So I undressed and joined her in the shower and she gave me the sponge to wash her back and washed it. Then she done mine and she then started on my front. This was to much for my cock and it went as hard as a rock pointing straight at her. She just smiled at me and continued to wash me with the sponge. On reaching my still hard cock she start to play with it and then went down me. I couldn't hold back and blew my cum into her mouth. To my surprise she swallowed it all and said let's continue this in my bed. So we slept together that night and every night since, after her daughter Grace was born we have continued sleeping together, except when the family visit. No one knows about Robyn and me fucking and Robyn has decided to stay on the farm.