My Mom And My Sister

I read on here a number of bisexual stories which I found very interesting. So I decided to add mine which is just a little bit different, I suppose.
Mom has been divorced for about eight years now. My sister is twenty-two and single and I am Twenty-four and also still single. The three of us have always gotten really well and I would say that we are a close family.
It was at the beginning of this last summer. I went over to mom’s place and I just let myself in the backdoor like I always do. When I went into the living room I was so surprised to see mom and my sister both naked and cuddling together on the sofa. Actually I was kind of stunned, just not expecting to see that. And of course they were not expecting to see me, either. They were both really embarrassed.
They each explained how they were just having a little fun together was all, and tried desperately to make it sound like it was no big deal. I could see how embarrassed they were and I didn’t want to make them feel worse, so I just acted very casual and normal about it, and said how that was fine with me.
Well they got up and tried to act just as normal about, sans clothes, and actually it really did seem quite normal and like no big deal. Later I thought how this undoubtedly had not been their first time doing something like this, and I could only wonder how long it had been secretly going on, and that obviously neither of them had any problem with being bisexual. I figured that it was just a mother-daughter sort of thing and I find it rather amusing.