Should we take in another Miniature Schnauzer?

Hi, need some advice please! We have a 6 year old Mini Bitch who has been spayed. We've had her for 3 years and we got her from Breeders, so she came from a home with lots of schnauzers. However, she is quite laid back, not the liveliest of dogs and isn't much interested in other dogs. We have been asked by a relative to take in THEIR 6 year old (not been spayed) Mini Schnauzer bitch. She is a lovely dog but we're concerned about 2 things: Will it upset our dog for another one to come on the scene? When she visits my Mum's dog she does perk up, but my partner is really concerned that her nose will be put out of joint. Also, we dog sat this other dog about 3 years ago and she kept urinating nearly every hour. Her owner said she didn't normally do that, so do you think this was as a result of pining for her owner? She would have a good home with us, but it does concern us about her being taken away from her owners. The only thing is, if it's not with us it will be someone else.
Any advice appreciated.