I think Brenna and Catherine might actually be supportive of this. In a previous chapter Cath suggested to Sterling that he should have sex with Brenna, and when he shot it down she pivoted to something like "Mom's been really lonely too." In chapter 11 when Sterling admitted to Kumiko and Catherine that he was fucking Ophelia, Cath asked if she could watch. Brenna is a little harder to read, but in her bad end with Drew in chapter 11, Brenna was super turned on by the revelation that Sterling was fucking Ophelia.
I also think Ophelia will be ok with Sterling fucking pretty much everyone. As of the end of chapter 11, I believe Ophelia is already aware that Sterling has had sex with: Eva, Mallory, Penny, Amber, Tiff, Aliza, Rachita, and Brenna. I can't wait to see Ophelia address the Brenna situation, as well as learn about Catherine and Lucia.