I am not liking what your breeder is trying to do to you.
Two dogs are great, but, two females can get you into a heap of trouble and not because either dog is bad or anything you might do wrong. It is just that two females, especially close in age will PROBABLY have issues with each other as they get to the point of sexual maturity.
The thing is your bitch and pup will do great together (probably) until the pup is between 9 and 18 months old. Then it will really depend on their make up. If the older/bigger dog is naturally more submissive and the younger dog is naturally more dominant in pack order, chances are there will be a change in status in the pack and it can be very ugly. Even if the older bitch is more submissive, she may not want the pup jumping on top of her, and exerting her dominance, and if she fights it will be bloody and it may not be the end of it either.
WHENEVER YOU GET TWO FEMALES THERE IS A POSSIBILITY THAT THEY WILL HAVE TO BE SEPARATED FOREVER. That means crate/rotate forever. And one minor slip up can mean a bloody fight where everyone (including the human) requires medical care.
If you have 6 or more years of difference in age and the pup is raised from baby puppy on with the bitch it may simply work out.
The thing is, you really can't tell, not at 8 weeks, not at 6 months, not at 12 months whether or not your bitches will get along. So if you are not willing to crate/rotate forever, then maybe it would be best to go with a male pup.
I think that if you like the yearling female, you should go for it, so long as the breeder is not going to charge an arm and a leg for her. The pee thing might get better. It may be something that you end up medicating for. I am glad that she is being honest about the problem. If you are concerned about that and your indoor dog, then maybe it is best to just pass on her.
RIP Arwen, CD RN CGC
RIP Whitney, RN CGC
Jenna, RN CGC & Babs, CD RA CGC HIC (not AKC)
Heidi, RA CGC & Tori, RN CGC
SG3 Odessa, SchH1, Kkl1, AD
Ninja, RN CGC & Milla, RN CGC
Joy, Star Puppy, RN CGC
Dolly CGC & Bear