well....11 weeks old is a bit young to give up on the pup, isn't it? i mean, you've had the pup for 3, maybe (but hopefully not) 5 weeks? don't give up, it is most definitely something you can stop.
here is what worked for my wife and i, which is right in line with what others have already suggested. the only reason i'm replying is because maybe it will help with your frustration knowing many other people have had the same issue - my wife and i have a dutch shepherd, also from strong working lines.
whenever teeth touch skin or clothes, on purpose or on accident, yelp and ignore. we did not isolate him in a different room (i'm not implying that isolating the pup is a bad idea, i'm saying we did not do that), we allowed him to try to get our attention, and each and every time he ended up in front of either of us, we would simply fold our arms and turn our backs - no eye contact with the pup while ignoring. when we did that, our dog got the point that mom and dad were really unhappy with him for some reason, and quickly put 2 and 2 together and figured out the reason why we were unhappy. it won't be an over night fix either, so don't expect that or get frustrated when you think you've made progress one day but all that progress seems to go out the window the next day. it'll get better for a while, then worse again, then better again etc as the pup matures and goes through phases where he pushes his boundaries or simply "forgets" things that you think have already been learned.
tug - definitely play tug. give that mouth something to do, something acceptible. tug is also a great way to remove some energy from the pup. 3 simple rules we used for tug were:
1 - you start the game (just because the tug toy comes out does not mean the pup is allowed to latch on to it immediately. pick a command to use to let him know he is allowed to start playing)
2 - you end the game (pick a command to use to have the pup release the tug, we used the method Elana55 suggested to teach this command)
3 - if teeth touch skin or clothes, on accident or on purpose -> game over...immediately, game over, toy put away, and ignore.
keep the toy you use to play tug put away, only take it out to play tug, and put it away when the game is over. keep this toy very high value and when you start to train obedience (or whatever) you have a great reward. our dog will do just about anything you ask him to do for a little bit of tug or a ball.
the more consistant you and your family can be with the reaction to the biting/mouthing, the better. this worked out very well for my wife and i because we are both very active with the training of the dog. whenever he mouthed either of us, we would both ignore him - do not allow other family members to comfort him or play with him while you ignore him for biting/mouthing you. you will have to work this out with the fiance and 4YO. definitely keep the 4YO and the puppy separate when you are not 100% attentive on the interactions between them.
being a GSD, your strongest leverage will probably be his desire to make/keep you happy - that was the case with our dutchie. he does not like it when mom and dad are upset with him. use that, it's a very strong tool. start thinking of a job for your GSD, as it sounds like he will most definitely need one.