I think I was 18 or 19 when I went with my mom on a vacation to Florida. She divorced my dad a few years before, so we were pretty much best friends. We have also always had a pretty open relationship, so when I walked out into the kitchen of our rental condo one morning, I wasn't bothered seeing her naked looking out the window with her hand between her legs. I stood next to her and asked what she was looking at, and all she said was 'holy shit, there are some sexy guys out there'. This was the 80's, so most of the guys' swimming trunks were pretty skimpy. But she was right, there were some really attractive ones out there walking around, swimming or just laying with their wives/girlfriends.
I pretty much said what the hell, then stripped off my shirt and underwear and stood next to her, starting to finger myself as well. For the next 10 minutes or so, we just watched the guys and made little grunting noises as we both worked towards our orgasms. Luckily the sun was shining the other way, so they couldn't see the mother and daughter, both bare-ass naked on their tippy toes, masturbating together in the kitchen. She came first, and when she did she made a sharp gasp, shuddered and stood there for a few seconds catching her breath. She then brushed her hair out of her face and simply went back to her bedroom. I wasn't far behind her, but when I came I let out a loud moan and had to sit down for a minute before getting dressed.
We did this a few more times when we were there, and eventually even went out and talked to some of they guys out there, but that's a different story.