Just

You can barely think the cum still lingering on you and some physiological need to eat that cum. The noises and sister's panic is barely processed. You know you should be panicking, you just had a random stranger blow a load on you but your brain just feels fuzzy and good... must be shock. Another effect seems to be time as you took 10 steps and your in some toilet.

Your sis recognises your shock still and grabs some tissues from an unopened stall. Helping clean up the parts you didnt lick off but she seems to get distracted by loud moans in a locked stall unsure whether or not this was safe. Fuck her lips look so good you thought, metaphorical salivating the part you focused on. You need to kiss them, you need to share the warmth and you know just how.