Is the marriage contract valid when the news of marriage to the first wife is concealed from the second wife's guardian, while the second wife is aware of it?

There is no harm in the marriage contract even if the guardian or the wife is unaware. However, it is preferable to inform the guardian if keeping the matter secret will lead to a problem. It is also advisable to seek guidance from Allah Almighty through Istikhara, as a person does not know what is good or bad for them. Allah Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah Knows, while you know not." (Al-Baqarah: 216).