Can I give the Zakah of money to my sister or brother if they are in need like preparing for marriage?

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.

It is lawful to give Zakah to brothers or sisters if they are poor. In fact giving it to them is more rewarded because of their blood kinship. It is reported in a sound Hadith that the Prophet (Sallallahu Alaihi wa Sallam) said: "Giving Sadaqa to a needy person is only a Sadaqa while giving it to a relative is both Sadaqa and a good act towards a relative" [al-Nasa'i].
So, it is lawful to give Zakah to your needy brother even if the latter will use the Zakah money to cover marriage costs and it is lawful to give it to your needy sister.
Allah knows best.