These girls have a very interesting dynamic. Miss Madeline is the more stable of the 2. By this I mean- she is predictable and sound in her behavior. Lucille is as I like to call her “My complicated girl”. She is dominant and insecure. She is moody- sometimes extremely over the top affectionate and other times- withdrawn and sullen. If she was a human, she would be diagnosed as Bi Polar.
So Lucille never accepts any dog in the home readily. It takes time. and even then, she has to remind them she is a force to be reckoned with. The funny thing is, she does this posturing when they could care less. Poor girl flaunts her ways and they are 
busy pulling out stuffing from the closest toy. Madeline was the perfect 3rd dog for this household. She is quiet and unassuming. She knows Lucille has “issues”, and she gives her space. Funny thing is, this relationship keeps on developing even after 2 years together. Yesterday, at the Albany bulb evening walk – they start sniffing each other and suddenly Madeline does a play bow and Lucille does a little dance! It went on for a few minutes and I got such a kick out of it.
They way I see it, Madeline is the smart one- she knows Lucille is a bit of a loose cannon- she knows she is “the special sister”- so she uses her quiet and respectful ways to gain Lucille’s trust. And her timing is impeccable- she knows exactly when to kick in a little play- it has to be when Lucille is totally relaxed and at ease with her- no food or toys in sight. I think Madeline would make a great negotiator or peacemaker- she is centered in her approach to Lucille- she knows exactly where Lucille’s boundaries are and never crosses over. and even when Lucille snaps and loses her cool, Madeline can immediately return to center and appease Lucille with a calming look.
Harmony usually exits between them. It may look like Madeline is the submissive one, yet- I see her as the wise one- giving just the right space and just the right connection to Lucille. Good Girl.