It is just so strange - how does she get relaxed enough to moan with her older daughter almost present?

With little children, sure, I had sex next to sleeping babies and toddlers in a family bed; they rarely woke up (children are usually not insomniacs), but on the very few occasions when the children woke up, my then H and I stopped having sex so that I could nurse the baby/child back to sleep, and then resume. It never caused any problems. But with preteens/teens?

Also, and more importantly, how did YOU, the grandmother, come to learn of this? Were you present, on one occasion, alongside both girls while the couple was having sex? Or did the grand-daughter TELL you about hearing sex? Because if she TOLD you, then it answers the question - surely she must be feeling uncomfortable enough to share her discomfort with YOU.