I m happy you told me you are in BKK Now am not worried about you.
The probleme is if they make love and she not eat medicin and he use not codoms maybe she get pregnant but not sure,.
You ought to tell your sister that, ( it´s dangerous to make love and not use contraception) and ask him if he like your sister. If he likes your sister you tell him USE CODOMS and make no baby. In Sweden and Denmark a girl may say it to a guy Don´t be shy it s normal.
Good luck tomorrow ! Tomorrow I thinking about you and your sister all the day.
When believe you are home tomorrow? I think you get very tired tomorrow.
See you later hugs and kisses