
I had a very similar scenario to you but only in the fact that i decided to get a male puppy to live with my 5 year old dobe bitch.My bitch was not keen on other dogs but despite all the advice I had on here telling me that under no circumstances should I introduce a new dog to my household, I went ahead and bought a 9 week old weim.I introduced them very carefully and allowed my bitch to go upstairs away from the puppy should he get too boisterous, which of course he did.For the first week she stayed upstairs out of the way, when she ventured downstairs I just pretty much let them get on with it, keeping an eye on the bitch in case she had a nip, which she did on a few occassions.The puppy always wanted to play and she wasnt impressed but after a few weeks she tolerated him.Then eventually she sucumbed and actually playes with him from time to time. I have had the puppy for 8 months now and although she will never see him as her best friend they live quite happily alongide each other.
I think that you can only try these things for yourself, you will get lots of conflicting advice but do what you feel is right.I expect your boy will find a puppy annoying, she wont be his best friend for a while,and wont replace his sister possibly but usually these things sort themselves out, with the older dog sorting out pecking order if you can stand to let him sort your puppy out for himself.I think that also as he is used to having another dog living with him and he will know its a baby, then it might be easier for him to adjust.You have done the right thing with the crate, and the muzzle, I did have both things when we brought our pup home.I think that if you try it and it doesnt work out then your breeder would probably have the pup back,despite the distance, the crate will keep them apart until you could take her back,but thats the worst case scenario.Hope this helps.Good Luck.