Yes

As dinner nears it's end, Fiona hasn't let up in her under-the-table activities, and Allison's face is red as she tries not to make any sounds.

You can tell from one look at Elsie that she knows exactly what's going on - there's no way she'd fail to realize it after seeing what Fiona was doing earlier.

As for your mother...she shows no signs of noticing, until abruptly asking Fiona in a deceptively casual voice. "Fiona, is there any reason in particular that you've had your foot between your brother's girlfriend's legs for most of dinner?"

You feel yourself stiffen in surprise, and even Fiona looks taken aback. Allison for her part looks completely mortified - in normal circumstances she'd probably never be able to look your mother in the face again after this.

After being caught, Fiona struggles to find words for a few moments, before finally snapping back as if she's not in the wrong at all. "Well, that's why she's here, right?"

To everyone's - varying degrees of - surprise, Fiona pushes her chair back, stands up, and strips the clothing off the lower half of her body in a few quick motions, continuing to speak as she does.

"Jesse invited his slutty girlfriend over so we could all shove these in her, right?" She shoots a grin at Allison's stunned looking expression, grabbing her exposed cock and waving it in Allison's direction. "Bet you can't wait for another taste of this, huh?"