I think it sounds more sexy for Rose to say she's Mary's sister.
"I can't believe I am fucking my sister's master"
"Wow, you would fuck your maid's sister? That's naughty, she probably would not be happy with that"
If you are into incest, it still works if Mary is your mother.
"I can't believe I am fucking my sister's child"
"Wow, you would fuck your Mother's sister? That's naughty, she probably would not be happy with that"
Probably even more sexy for incest, because it's less straightforward, and makes Rose sound younger (aunt has the "I am old" connotation. Some probably prefer that I guess, *shrug*). Sounding young is fitting because Rose is very lewd and doesn't hold herself back as much.