Lordcross is right i am also wondering what is wrong with the logic here. Not to mention the morals of the older sister... something is definitely wrong here.
Isn't she more than likely the boys' babysitter? Onee-chan can refer to any woman older than them. Pretty much same as Ojii-chan here. I'm assuming the man is the boys' father and they're having an affair or something.
H.. He's cumming inside her?!C'mon, my turn...Ah...You're filling me up with your cum... ♥Ahn... ♥Ahhh... Chie-chan, your pussy is just so tight! I can never last long with you.But I'm not done with you yet. I'm going to cum inside you so much today!If you two behave yourselves, as a reward...I'll let you secretly watch your big sis having sex with Oji-san.
It's unclear who her partner is. "Oji-san" could mean just about any grown man older than the speaker.