Is it permissible to conceal from the groom the manner in which the bride was proposed to, and is that considered a forbidden lie?

If the groom's sister knows nothing bad about the fiancée and perceives her to be upright, she is not obliged to inform her brother of the means by which she came to know her, and there is no sin in her concealment. If her brother asks her and she fears a negative consequence from informing him, she may use dissimulation (tawriyah) without outright lying.