The defendant files an appearance in order to notify the plaintiff and the court of the location where further notice may be had upon him/her. It is necessary, usually along with a response, in order not to be defaulted. You should not be concerned about it but be grateful the defendant has been served and has appeared.
Unless your sibling is a defendant in the suit, it seems odd that he or she would file an appearance. Of course, if its your sibling, you could just call and ask why they filed.
Practically, it means you don't have to chase them around with a process server, they have to answer your complain within weeks and have waived the objection to personal jurisdiction.
No need to be concerned. I'd be concerned if it did not occur in a timely fashion. An appearance is a perfectly common component of litigation.
The author provides the preceding information as a service to the public. Author's response, as stated above, should not be considered legal advice. An initial attorney-client conference, based upon review of all relevant facts/documents, will be necessary to provide legal advice upon which the client should then rely.