Concerning Zakaat Al-maal, I have the following questions:
1) My mother has her own income but last year she wanted to treat her teeth and she didn't had the money, I have sent her Euro 1000, can I consider this money as Zakaat or not?
2) My brother is still studying and is not working, my mother is responsible for him, is it eligible to give Zakaat Al-maal to him?
3) I didn't paid Zakaat Al-maal for 3 years now, do I still have to give it , or should I start from this year?
4) What is the Nisaab for Zakaat Al-maal in Euro.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

It is not acceptable to consider what you have given to your mother as Zakat because Zakat cannot be paid to parents. Many Fataawa have been issued on this regard, so you may refer, for example, to Fatwa: 84019. As regards your brother, if he is among the people who are liable for Zakat, then it would be permissible to give him Zakat. The kinds of people who are liable for Zakat are mentioned in Fataawa: 88368 and 86546. The ruling of giving the Zakat to a brother is mentioned in Fatwa: 82645, so refer to it.

With regard to the Zakat that you have not paid for the last three years, it is still a financial obligation upon you, which you have to pay immediately, and repent to Allaah because delaying the payment of Zakat without a sound religious reason necessitates repentance.

The minimum amount liable for Zakat in Euros [or any other currency] is the value of 85 grams of gold; so whoever possesses that amount, possesses the minimum amount liable for Zakat, so after a whole lunar year with such amount in your possession Zakat has to be paid on it. Please refer to Fatwa: 85396 for more details. 

Allaah knows best.