Step daughters/Asexual/Shower both
"Well since you're used to shower together let's keep it that way. Plus it'll save us some time" You say, more than impatient to see the beautiful Masha naked;
Their mother Maria is happy you backed her up and goes to the kitchen while you get both girls in the tub.
"Come on, time to shower" you say.
"Sandra, do you need help to undress?" (You doubt it but girls her age sometimes like the extra attention)
Sandra: "Oh yes please!"
So you help her remove her top, getting as much physical contact as you can while watching Masha undressing in the corner of your eyes.
Once Sandra is naked you pat her gently on her ass and turn toward her sister. Masha is sideways and you can see her perfect ass and pretty face from where you are. When she turns to look at you also get a great view of her pink slit where you eyes remain a few seconds to many while you get an instant hard on.
You can see Masha's face turning red and her eyes go shortly toward your pants, suddenly way tighter. She makes a vague motion to cover her slit with her hand but you take her hand away as you guide both girls toward the tub.
Once they are inside you check the water is at a good temperature and shower Sandra slowly, on hand on the shower head and the other on her at all time. When the water spray reaches between her leg Sandra starts to giggle. "That tickles" she says.
"Oh really?" You keep playing with the water spray on her vagina for a while, making noises and faces, Masha is blushing really hard.
Eventually she is completely wet and you pass the spray to Masha while you start putting soap on Sandra.
You have your hands on Sandra's back when you realize Masha stops with the water spray between her legs as well. With a huge grin on your face you keep cleaning Sandra and reaches her inner thighs with the soap.
Masha sees you and tells you "you're not supposed to touch there!"
You:"But I am, this is very important to clean yourself there and who else would do this? You, you want to put your hand on your sister's vayaya?"
To Sandra: "I'll need to clean your vajaja now, I know it is very sensitive so I'll be careful but you can tell me if it hurts ok? Do you understand?"
Sandra:"ok"
So you put your hands in between her legs and start very gently to rub her clitoris already warmed up by the water spray earlier. Sandra giggles happily as if you were tickling her but soon her giggling becomes less random and more like her rubbing herself on your hand. Without thinking you take your fingers to your lips and lick them to help lubricate even more. Masha goes "Errrrrk" but Sandra does not react and soon you put your hand back between her legs. She rubs herself against your hand while Masha looks with a weird mix of fascination, disgust and curiosity and yer eyes go often toward your hard on (covered by your trousers). Sandra becomes more and more precise in her moves and starts to moan lightly right before getting stiff and then completely relaxed. You finish cleaning her up quickly just in time to hear Maria say "Breakfast is ready!"
You take her out of the tub and dry her up while Masha is taking care of herself.