Yes. I'm bilingual french/english, French being my first language. I still stutter a bit more in french compared to English. My speech therapist once told me it can happen when people learn a second language after being taught avoidance techniques in their first language. In other words, since I've known techniques to deal with my stutter in french before learning English, my brain kind of does it on its own when I speak English. Also, the novelty is a big factor. People who stutter often say that doing an accent or a different voice will decrease their stutter dramatically. The thing is, if you do it all day everyday it eventually comes back. That's because the novelty wears off and the brain falls back into its normal patterns. Maybe that's what going on with your wife's sister since you said shes rather young and still learning the language.