Can I give Zakah to my real sister who I think deserves help from Zakah but without telling her it is Zakah? If I tell her that this is Zakah money then she may refuse accepting it.

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.

You may give your sister the money with the intention of Zakah. And then you would become free from your obligation. Also, it is not mandatory for you to inform her that it is Zakah since informing a beneficiary about Zakah is not required in Shari'a.
The jurists have two different opinions concerning the validity of giving Zakah to relations.
However, the preponderant opinion of Muslim scholars from the period of the companions of the Prophet (Sallallahu Alaihi wa Sallam) and their successors is that giving Zakah to relatives, except the father and the child, is permissible. The evidence for this opinion is the many proofs in Shari'a that encourage one to give charity to relatives. For example, the Prophet (Sallallahu Alaihi wa Sallam) said: "Sadaqa (charity) given to a poor man is just Sadaqa, but when given to a relative it serves a double purpose, being both Sadaqa and connecting kin relations" . [ Ahmad al-Nasa'i , Ibn Hibban and al-Tirmizi who classified it as Hasan].
Also, the verse in Qur'an asking that Zakah be given to the poor no differentiation is made between relations or others Allah Says (interpretation of meaning): {As-Sadaqât (here it means Zakât) are only for the Fuqarâ' (poor), and Al-Masâkin (the poor). …} [9: 60].
The Hadith of Muadh is also general in meaning. The Hadith reads: "(Zakah) is taken from their rich people and distributed amongst their poor people" .(al-Bukhari)
All the above and similar text of Shari'a includes the relatives and there is no evidence that proves otherwise.
Allah knows best.