I'm going to be completely honest with you. I am a total cavy slave, I love my guys to death. However, several weeks after getting my two boys, they had to get out of the house. Yes, in some people, the allergies aren't so bad and it can be controlled with medication. But is your dad willing to take medication every day and willing to pay for the expense of constantly buying meds? Guinea Pigs live a long time. The allergies, in my case, were really bad. It started just when I touched them, my eyes itched and turned red. Then, my skin started breaking out and itching, my eyes started swelling shut. The longer I kept them, the worse it got. The reaction would take several hours to go away. I tried making sure not to touch my face etc, and washed my hands, but it didn't seem to matter. Then it started affecting me just when I walked in the house. Sneezing, itchy eyes, swelling eyes, etc. I have two kids to take care of and had to make them a priority, so Peanut and Butter had to go. Lucky for me, my sister took them in.
Obviously my allergies are worse than most. I've never been allergic to animals before, but it's not uncommon to develop allergies later in life.
You have to think, is it fair to your brother to get them, and only have your dad be unable to handle the reaction and your brother have to give them away? Wouldn't that break his heart more than never having them? It depends on what dad is willing to do/go through. I'm a total advocate to getting the piggies out of shelters/rescues and into homes. But I also don't think a rash decision is wise. They might just end up right back in a shelter if you aren't sure that dad can deal with this.
I have to take crazy amounts of meds to visit mine at my sisters house, and can only stay around them for a few hours.