The father has died. May the mother act as her daughter's marriage guardian?

According to the Shafi'i school and the law in Malaysia, a mother cannot be the marriage guardian; guardianship passes to the paternal grandfather, brothers and so on, or to a judge-appointed guardian if there is none.
The Prophet ﷺ said there is no marriage except with a guardian (Sunan Abu Dawud 2085), and in the Shafi'i school the guardian must be a male agnatic relative on the father's side. This does not lower the mother; her role in choosing and advising is very important, and her blessing and views should be heard. At the ceremony, the mother may sit beside her daughter, give advice and pray. If a brother is still a child or there is no eligible guardian by kinship, the religious office will determine the right guardian, including a judge-appointed one. See the order of guardians.