The sisters' friend is most certainly going to sabotage any excuse they can somehow eek out.
I can see the friend help keep the secret just so they're in debt to her + she can keep toying with their relation (which was the whole reason she intentionally get clingy with lil bro like this)
If she only care about getting reactions out of the two she might even help steer the sis from this double suicide path.
Like imagine her telling sis that 'hey, if your mom break you apart I'd have him all to myself, you sure you want that?' and I can see the sis stop this rape threat.