A friend of mine had a proposal of marriage. She asked me if I knew the groom. It turned that I know the groom's sister. Unfortunately she is not a good lady. Should I mentioned that to my friend? If the prospected groom himself is not a good gentleman and not suitable for my friend should I tell her that or should I keep silent and pretend not to know anything? Please help me in this matter, for I have difficulty telling lies, even my keeping silent for I believe that this is a testimony and I should say the facts only. I do feel uneasy and guilty when I say bad things about people even if they are true. Thank you for your kind assistance.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We pray to Allah to reward your commitment to piety and maintaining your tongue from backbiting others or spreading calumnies among people.
Basically, a Muslim should not backbite anyone. One exception of this rule is stated under the title of "permissible kinds of backbiting" in al-Azkar book of Imam al-Nawawi as follows: 'If someone seeks your advice concerning entering into partnership with somebody else, relationship in marriage, entrustment and similar dealings and transactions, it is your duty to make mention of what you know about him as a form of advice. In simple words, if the goal is achieved by simply saying: 'It is not fit to deal with him or to have relationship in marriage or do not do such an action and so on', then it is not permissible to say more, i.e. to refer to his defects; otherwise, you can specify his state of affairs in candid words' .
The above-stated opinion is proved in the sound Hadith recorded in Sahih Muslim that reads: " Fatimah Bint Qais told the Prophet (Sallallahu Alaihi wa Sallam) that Mu'awiyah and Abu Jahm had asked for her hand. Then, the Prophet (Sallallahu Alaihi wa Sallam) said: " Mu'awiyah is poor, and Abu Jahm often beats women" .
Thus, it becomes clear that it is your duty (if you are certain) to inform your friend about bad qualities of the mentioned person (i.e., a fiancé and your friend's brother), but avoid mentioning anything about his sister or brother.
Allah knows best.