Your intuition points to your argument as triggering the dream. Your perception of your mum as lovely points to her value as a wise and truthful response to to your argument. So that leads to the question:what does your mum's crying mean? Could it be sorrow that your niece has taken financial advantage of you? Could it be sorrow that you and your niece are in conflict? Could she be feeling sorry for you? When she was alive, how did you normally respond to her tears?
Imagine you are feeling HER emotion in the dream, not yours from the argument before it. What could those tears advise you to do? If you get an intuitive feel for that, or you can imagine her speaking the mind that is shedding those tears, then you have advice from your mum's afterlife perspective. But you don't have to carry out her response, even if you're pretty sure of what it would be. Because dreams allow your unconscious self to speak with your conscious self (ego), but they don't mean that you must choose your unconscious direction over your conscious direction. You can weigh the options, including some compromise that includes both points of view. Dreams offer you the chance to respect both sides of your personality, so you are a wiser person for gaining access to both sides of yourself. Since the dream came promptly after your argument, it suggests that having both perspectives is very important for you. And so, too, is improving the contact between your conscious mind (ego) and your unconscious (other characters and parts of your personality like your mum) is a benefit to your present coping and your spiritual development; and that contact will grow stronger if you carefully consider your options when taking both perspectives into account.
Were there ANY OTHER elements in the dream? Like the room, the bed, your mother's apparent age, what she was wearing--what was especially making her look lovely? Your own emotional reaction?
If you imagined why she'd be crying for herself, what would her feeling & thinking be?