Thank you for your Question.
According to Islamic laws, Na Mahram should not have any relation with one another. Your relation with this sister is Haram and should be anyways nullified because of the same.
Ethically a promise should always be kept. And in this case, since you promised to be life partner earlier on, it is only fair to let her know what your stand in life is. Be it something as simple and lay as finance! (assuming you’re sending an official proposal after paragraph one)
Your straight and polite communication will open new horizons for both of you. If she then finds it difficult to understand you, then maybe the relation was never meant to be.