Praise be to Allah.
If this girl is religiously committed and of good character, and you have prayed istikhaarah asking Allaah for guidance with regard to proposing to her, then go ahead and do not hesitate, for whatever Allaah makes easy for you will be good, in sha Allaah. The sign of istikhaarah is when a thing is made easy. If it becomes difficult and does not go ahead, that is a sign that there was no good in it for a person. It says in the du’aa’ of istikaarah:
“O Allaah, if in Your knowledge, this matter (then it should be mentioned by name) is good for me in my religion, my livelihood and my affairs, then ordain it for me, make it easy for me, and bless it for me. And if in Your knowledge it is bad for me and for my religion, my livelihood and my affairs, then turn me away from it, and turn it away from me, and ordain for me the good wherever it may be and make me pleased with it.”
The fact that this girl treated your mother badly is not a sufficient reason for leaving her. That may have happened because of heedlessness or it may have been a mistake, or due to excessive shyness and so on.
But if it becomes clear to you that this behaviour indicates a fault in her religious commitment or character, then in that case your hesitating to go ahead with the marriage is justified, but if you want to go ahead with the marriage, you must set things straight between your family and your fiancée, and prepare your family to welcome a new member.
Conclusion: whoever is sincere in turning to his Lord and delegating his affairs to Him, and prays istikhaarah for guidance in some of his affairs, should go ahead and do it. If it becomes easy for him, this is a sign that it is good for him, but if he is stopped and it becomes difficult, then he should forget about it, and he should realize that being prevented from doing it is good in sha Allaah. May Allaah help us and you to do that which He loves and which pleases him, and may He may the best of things and those which are dearest to him easy for us.
And Allaah knows best.