I personally think they are all myths and u have a 50/50 chance of boy or girl but who knows
Oh and as for the twin thing you can't control how many eggs are released unless you have help from a doctor other than that its just a 50/50 chance and you'll have to cross your fingers and hope for the best! But I believe the doctor can give her some kind of fertility pills that increase the chances if multiples.
I heard if you have sex closer to her egg release time (ovulation) your likely to have a boy. If you have sex days before ovulation I heard the girls sperm live longer and your more likely to have a girl. It could be sn old wives tale but I will say we didn't have sex for a few weeks before I ovulated an then had sex on the very day I ovulated and had sex everyday after and Im FINALLY (lol) having a Boy! I have 3 girls and with them we didn't have sex the day of ovulation it was always days before. Not this could still be an old wives tale a purely coincidental but im gonna stick with it worked for me! Good luck!