My father died 11 years ago. My mum deposited 5 thousand pounds in the bank (as Shadat istismar) after his death. And now, I am 21 years, so it's the age to get it.My Question is this amount of money 26 thousand now considered as interest or not because of the circumstances, or the 5 thousands only are Halal ?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If this 'shehadet estismar' constitutes a depositing of money for a yearly fixed profit with guarantee of the capital, then this is Riba (usury and interest) and Riba is completely forbidden in the Quran and the Sunnah.

The Prophet said: “Allaah has cursed the one who consumes Riba (i.e. usury or interest), the one who gives it to others, the one who records it and the one who witnesses it (its transaction).” The Prophet said: “All of them are equal in sin.” [Muslim]

Then, you have to take your original capital (5,000 pounds) as well as the interest. But, you must get rid of the interest by spending it on the poor and needy, or on charitable projects but not with the intention of giving charity.

Allah says: {O you who have believed, fear Allaah and give up what remains [due to you] of interest, if you should be believers.) And if you do not, then be informed of a war [against you] from Allaah and His Messenger. But if you repent, you may have your principal-[thus] you do no wrong, nor are you wronged.} [Quran 2:278-279]

However, if you are poor and what you possess does not satisfy your needs, then you can benefit from this interest according to your necessity only.

Allah Knows best.