I love my 18 year old step-sister's snot as lubricant

Pretty 18 year old step-sister wants to see her step-brother masturbate, but she can't, disappointed she decides to ask him to let her see, to which her step-brother asks for help with that, she proceeds to give him a deep throat blowjob, but his penis is very big for her throat, so she gags and snot comes out, but he asks her to keep sucking until he cums in her pretty mouth.