Family is best

I'm a 61yrs old male a bit over weight divorced and was living alone on my 200 acre mainly pig farm. My wife divorced me 12 yrs ago and she remarried 7 yrs ago. We get on better now we are divorced, but hardly see each other. Except at family events, our 3 children are all grown and have families of there own. March of last year my sister Susan 56 came to visit at my request, as she was having a lot of troubles (financial and personal). Her Husband Malcolm had killed himself, when his business collapsed and he couldn't cope. After the liquidators had finished. Susan had no home, no money and no prospects. I paid for her to come and told her I would help get her back on her feet. She arrived on the fourth and I told her she could keep my house tidy and such. I would give an allowance of $200 a week for doing that, while she looked around for a better job. She also had the use of my car, as I had my truck. Her 2 children (Ted & Mike) work in finance on east coast and weren't doing anything to help her. All was going well and she was returning to her normal happy self. She had applied for a couple of jobs and missed out, she had plenty of spare time as there was only the 2 of us and we were both tidy by nature.
We went out to the movies and for dinner a couple of times. Then on a Wednesday in May, I was moving one of my 3 diary cows Sara, Susan was with me. I put Sara in the bull Ramrod. Ramrod was quick off the mark and was soon covering Sara. As we watch them, Susan said "I miss getting fucked the most". I replied " So do I and I got too much work to do around here to go chasing skirt". As we walk back to the house Susan said " I'm here and I'm too old to get pregnant and we get on very well together. We could satisfy our needs and no one need know". I was a bit taken aback at that, but was also a bit aroused by it. I said " We couldn't it would be wrong". She replied " Why wrong, it's not like you are forcing me against my will and it won't hurt anyone and it's none of their bloody business anyway. It's just an idea and I'm not forcing you into it.". I said " I know your not forcing me and I can't say that I'm not tempted". Well she replied there no time like the present. So we went into into my bedroom and gave it a go. We were there for about 3 hours and after a slow start we got going and we tried out each others fantasises. We settled in to a routine and sleep together everynight and have sex 2 or 3 times a month. Susan has become my wife in all but by law and she not looking for a job and is helping out around the farm. No allowance for her anymore, she has full access to my bank accounts and even does my farm accounts for me. She changed her surname back to her maiden name. Lot of people who don't know we are brother and sister think we are married.