Stacy, 
 
Thank you for the questions. I'll try and answer both of them. 
 
Just a point of clarification - all worthy members (12 years and older) of the Church can enter the temple and do some limited temple work like baptisms for the dead. This is a very positive and sacred thing that the youth of the church do on a regular basis. But most of the rest of the temple work (i.e. endowments and sealings) are reserved for worthy adults. 
 
So when you say the Highest Priesthood - I'm assuming you are talking about the Melchizedek Priesthood. 
 
For brethren to participate in the rest of the temple work (being sealed) they need to have received the Melchizedek Priesthood and be worthy to receive a temple recommend from their Bishop. 
 
Quote: "I know when we do the work for the dead, they are sealed without having the Priesthood.". Actually part of the whole proxy work for the deceased also includes receiving the priesthood by proxy - so the dead actually do have the ordinance performed for them. And then it is up to the deceased individual to either accept or reject while they are in the Spirit World the ordinances performed for them by others by proxy. 
 
Sisters only need to be worthy and receive a temple recommend from their Bishop.