The my sister told that she is going to a party <b>at/that is at/which is at</b> her friend's house.<br/><br/>He is going to the seminar <b>at/that is at/which is at</b> his brother's office's seminar hall.<br/><br/>I'm going to a summer party <b>at/that is at/which is at</b> my bff's house.<br/><br/>Can I just use <b>at</b> or should I use <b>that is at or which is at</b>? Please help me understand.