Shia man is allowed to marry a Muslim woman from any Muslim sect as far as such marriage will not harm his faith and religious practice. If your mother is disagreeing, then you need to do everything you can to convince her and make agree without forcing her. You should never cause anything which hurts the respect or the feeling of your parents.
There would have been no harm in you congratulating her for her wedding, even though you are in the sacred mourning month of Safar. It is not haram to have done so, as long as you are not intentionally violating the sanctity of the month by engaging in celebrating behaviour, and meeting and congratulating her would not have been such.
In general our esteemed Maraji' permit a female who meets the shar'i requirement of being a balighah rashidah, to marry without a need of a guardian's consent, as long as the condition of her being so is correctly defined and understood.
It is not just a matter of maturity, but that she has absolute independence in her affairs, not just financial, but social as well. And that she is completely consciously aware of what is good and what is bad for her, and is competent in her decision making.
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