We have five month old boy (Apollo) and brought home our second boy (Arion) a week ago (he is 8 weeks old). We decided to bring home a second because we wanted Apollo to grow up with a sibling.
We did a lot of research before deciding which (male/female) to bring home. We decided to bring one home before Apollo started to get the "little prince" syndrome. Yes, two puppies at once is a lot of work; however we are fortunate enough to have the time to fully invest in our boys.
On the three hour car ride home, Apollo wouldn't even look at Arion. Once we were home, Apollo wasn't as indifferent but still a bit aloof. They don't fight but Apollo is still a puppy and doesn't know how to fully control his strength so to speak. I don't take my eyes off them for a second. Apollo gets a lot of alone time with us when Arion is asleep. Arion is being crate trained (doing spectacularly) but we try to get them to sleep in the same crate at least once a day. I think it helps to form a bond.
I copied and pasted the two links because getting a boy (since you have a girl) might help to minimize the fighting issue while alleviating the jealousy issue.
Definitely helped us in making our decision.
Male vs Female Yorkie's Boy o rGirl Yorkie
Every situation is different but I believe it's better to go in knowing as much information as possible; therefore, I decided to share my story.
Good luck and I do hope you find a wonderful sibling for Sophie~~
Sent from my iPhone on the way to the vet~~hubby driving **wink**