Our piano was in the middle of our apartment in Hong Kong. I remember all the doors to every room closing as soon as I started to practice. Being the eldest of three, I do not recall giving much real help to my younger brother and sister in my teenage years. I recall telling them things were wrong, and showing them how to do them "correctly". It wasn't until later that I found out I probably put them off: and that they might have felt pressure I never experienced before (because I was the eldest, and there was any older sibling to compare against). So in some ways I think I can relate to what you are going through. Except you are probably a far better pianist ...
I played in a trio with a violinist and a cellist around that time, and I was often told I was too loud. I could have taken that as helpful advice, and tried toning down; but most of the time I would start playing softer, and then bring the volume right back to when I started. I guess I did not like being told what to do ... Playing on a grand piano in a small room was not helping either.
On occasions when we all talked about the music and what kind of effect we wanted to achieve, I normally would try (willingly, not begrudgingly) and play with a bit more "sensitivity" ... : )
I had fun playing four hand music with friends. Perhaps you could suggest learning / playing something together with her. If could be for a particular occasion (something to work towards). As you start to play with dynamic contrast, she may very well follow ... Similarly about playing in time ... laugh about the mishaps. Or even make a video of them just for fun.
The main idea is for both of you to have fun with the music, work at something together as opposed to risking putting her on the defensive (when really you are trying to be helpful, but she may not have the maturity to not take it personally).
I am not sure if this helps at all but good luck anyway.
Topic: Quitting Piano