Keep giving girl a UTI?

My sister had this problem... you two probably have a PH balance problem. It happens sometimes. Try using condoms WITHOUT spermicide or lubrication. Your PH is different than hers and it is causing a problem in her vagina. Also try washing your penis before having sex, DIRECTLY before having sex. Also tell her not to douche if she does because this washes out healthy bacteria as well as bad and thus opens her up for infections. Hope that helps :D