Is a wife who took from her husband's money without his knowledge to cover the expenses of their son's marriage, due to her inability to inform him, sinful, and is what she did considered a Sharia violation?

If the husband spends according to what is customary, then the wife is not entitled to take anything from his money without his permission. The wife must return what she took without her husband's knowledge and consent, or seek his halal (permission/forgiveness) for it. If she fears a negative consequence, she can return the money without informing him.