Use your fingers.

I take a breath to steady myself then watch as Jane reaches down and spreads her pussy out to me. I touch the nub, which she later said was the coltis, and I play with it for a minute then see her 'pee-hole' and the larger hole under that.
My reason screamed for me to not do this...but my inner-demons told me I wouldn't get another chance.
Hand shaking a bit, I stick in one finger. It was warm inside there! I keep going until I hit something hard. I begin to pull back when Jane started to moan...what the heck? I stick it back in...then out.
She kept doing it.
I wiggled my fingers around inside of her.
Jane began to squirm on the bed.
I stopped and she panted. "How...How did it feel?"
"No lumps." I answer.
Jane then pointed to the 'bird beaks'. "Now use one of those to look."
I bit my lip and did so once she told me how to use it. I open my sister up and shine the penlight inside to see...something.
"So...what am I looking at? A ball?"
She giggles. "No silly. It's the entrance to my womb."
My eyes bulge. This was...?
Quickly I withdrew it and wash it in fresh ****. My mind was reeling.
Jane looked at me in concern. "You ok?"
"Y-Yeah...just shocked that I saw my sister's insides."
She got off the bed and put on her clothes. "Did it...disgust you?"
I shook my head. "No...nothing like that. You should-"
"You guys up here?"
We both turn to see Chris coming up the stairs. Jane goes out to stall her while I put everything away in the old doctor bag. I come out to see them talking.
"Brings back memories being up here, huh?" I ask.
"Yeah, it does. What were you both doing up here?
Jane waved her hand, "Looking through the trunks...talking...actually we found some really old Medical charts from the 1600's."
"Really? Cool!"
"Yeah. Bring the girls up and we'll look at them together."
Chris leaves and Jane, sensing my panic, says, "Don't worry. I know what to do. We'll get them horny enough to want to play. And if that doesn't work..." She gives me a grin. "I'll use something I've saving for a 'rainy day' like this."
Why did I suddenly feel scared?
The girls come up and we look through all the old charts, but the mental image of sister being examined by me still replayed in my brain.
"I have an idea! How about we play Doctor?" Jane asked. "We can use the stuff we found."
"I don't know, Jane." Chris said. "Our folks might be back any minute-"
"It'll be quick."
"Great idea!" Francine said, crossing her legs. "I haven't played that since I was five."
"I agree!" Janet piped up. "Who'll be the Doc?"
"John will."
Only Francine and Janet seemed to like that idea. Chris knew she couldn't go against three...so she agreed in dismay.
We decided to draw straws to see who'd go first. As Chris went to get them, Jane said to me, "Now listen John, don't tell Chris, but we're going to play this game differently."
"How so?" I ask, already knowing the answer.
"Yeah, Jane. How so?" Janet repeated.
Jane sat back on her hands. "You John...will examine every inch of us." She patted her pussy. "Including down here."
Francine and Janet giggled before agreeing.
"I'll be the nurse, in case John needs help."
Chris came back and I went inside to 'prepare'. I laid out the tools, put on the stethoscope and poked my head out as the first girl came in with Jane behind her.