Is this female relative considered a sister by rada'a (nursing) in the case of doubt regarding the number of nursings, and is it permissible for a man to communicate with his non-mahram female relatives?

If your father's wife breastfed you five times or more from milk produced by your father's cohabitation, then she is your foster sister. Ignorance regarding the number of feedings occurs in two forms: First: If you know that it was more than five but do not know the exact number, then this establishes prohibition. Second: If you do not know whether it reached five or not, then this does not establish prohibition, but it is more cautious to avoid marrying someone about whom there is doubt regarding her being a foster sister. Maintaining kinship ties (silat al-rahim) is obligatory. For relatives who are not unmarriageable (mahram), such as a paternal uncle's daughter or a maternal uncle's daughter, kinship ties are maintained by sending greetings and fulfilling needs without any forbidden acts. However, for those permanently unmarriageable, such as a foster sister, it is not forbidden to be in seclusion with them or shake their hands if there is no fear of temptation (fitnah).