Is not knowing the aunt's place of residence and phone number considered a legitimate Shar'i (Islamic legal) excuse for not visiting her and maintaining ties of kinship with her?

Maintaining kinship ties is one of the best acts of devotion and greatest acts of obedience, and it is obligatory, while severing them is forbidden. Since religious obligations are tied to ability, the questioner should exert her utmost effort to communicate with her relative and maintain ties with her in any possible way if she knows her location or a means to contact her. If that is not possible for her, then there is no sin upon her, and she should frequently supplicate for her relative's well-being.