What is the appropriate stance to take towards the husband and family given the change in the sister's fiancé (the questioner's brother-in-law) and the change in the fiancé's mother (the questioner's maternal aunt) towards the sister, with the suspicion that the cause of this change might be sorcery, and is it advisable to seek the help of a shaykh?

You should search for the real reason behind the estrangement between your sister and her fiancé and his family. It might not be magic, but rather other reasons such as bad talk about you or your sister's behavior. If it becomes clear that there are no natural causes, then the reason can be investigated. If it is magic (sihr) or the evil eye ('ayn), then it is permissible to treat it, while being cautious not to go to sorcerers and diviners or to use polytheistic incantations (ruqya). One should resort to Allah Almighty and be persistent in supplicating Him to facilitate the matter and remove the distress.