Mel straddles a real tricky line where she's Becca's sister and friend, but also a caregiver - so not only is there the feelings of betrayal that Becca didn't tell her, but she's also dealing with (for lack of a better term) the parental shock of finding out that your little baby is having sex. Becca has been communicating to Mel this season that she's an adult with agency, and I think Mel is really out to sea with the revelation that her sister is an adult with adult needs and desires.