Assuming that there are no obvious impediments to a valid marriage, yes. As your niece has formally renounced her Catholicism by her entrance into a non-Catholic church, she is no longer formally bound by Catholic marital law. If she is marrying another baptized Christian, the Church would presume her marriage to be both valid and sacramental. If she is marrying an unbaptized person, the Church would presume her marriage to be valid but not sacramental.
