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.