A nun in Spain is enjoying the media limelight; probably a bad thing (look up what happened to the Singing Nun). So she's shooting her mouth off: "Mary wasn't a virgin!" This is from a tabloid but still.
I'm not her bishop but I would start the proceedings against heresy. Warn her, and if she persists, throw her out.
Our holy mother the church doesn't handle heresy like our Protestant hosts in Britain and America think we do (the popular notion about the Inquisition with torture, the Black Legend of Spain, and all that). Historically we don't kick out everyone who has wrong views. Goofy Aunt Terri who thinks a woman should be Pope is fine. Even the Pope's opinions can be wrong; his job (all we care about) is limited to defending our doctrine. To be a heretic you have to 1) be in a position of authority and trust so you could do harm, such as a priest or professor, 2) be in a position to know better, such as someone with a Catholic theological education (we don't punish the ignorant), and 3) be warned; the church doesn't sucker-punch you with excommunication. And even excommunication isn't what you think. We don't issue tickets to hell; we're not that arrogant. It's a warning that you're in grave error, asking you to come back.
Fulton Sheen: People don't hate the Catholic Church; they hate what they think we are.