She took some money from her mother when she was still alive without her knowledge, and now she is the only heir

If a child takes from the wealth of the one who is obliged to
spend on him, whether that is the father or the mother, one of two scenarios
must apply: 

(1)

He takes what he needs to meet his need for food, drink,
clothing and so on. This is permissible, even if it is done without the
knowledge of the person whose money it is, if he has no way of getting his
rights to maintenance except in this manner. That is because of the report
narrated by al-Bukhaari (5264) and Muslim (1714) from ‘Aa’ishah (may Allah
be pleased with her) who narrated that Hind bint ‘Utbah said: O Messenger of
Allah, Abu Sufyaan is a stingy man and he does not give me enough
maintenance for myself and my children, except what I take from his wealth
without his knowledge. He said: “Take from his wealth whatever is sufficient
for yourself and your children, on a reasonable basis.” 

(2)

He takes more than he needs for his maintenance for the
purpose of having more. This is not permissible and it comes under the
heading of taking wealth in an unlawful manner. In this case he has to
repent and return the money to its owner if he is still alive or to his
heirs if he is dead. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was
asked: I am the only daughter of my father, and my father – praise be to
Allah – is very well off. Sometimes I take from his money without him
knowing and he does not ask me about it. Am I sinning by doing that…? 

He replied: It is not permissible for anyone to take anything
from anyone else except in a lawful manner. If this girl used to take money
from her father’s pocket because she needed it and if she asked her father
for it he would not give it to her, then there is nothing wrong with that
because Hind bint ‘Utbah asked the Prophet (blessings and peace of Allah be
upon him) [about such a matter]; in fact she complained to the Prophet
(blessings and peace of Allah be upon him) that her husband did not give her
enough for her and her children, and he said: “Take from his wealth whatever
is sufficient for yourself and your children.” 

But if the father of this woman who is asking the question
did not withhold anything from her if she asked him for what she needed,
then it is not permissible for her to take anything from him without his
knowledge.

End quote from Fataawa Noor ‘ala ad-Darb

For more information please see the answer to question no.
83099 and
149347 

Based on that, you have to repent to Allah, may He be
exalted, for what you have done and feel remorse for that. You do not have
to return the money unless there are other heirs who inherit along with you,
because they have a right to this money. 

Offer a great deal of du‘aa’ for your mother, because that is
part of honouring her after her death. If you give some of the wealth in
charity on her behalf, that is the hope that it will reach her and will be
part of your repentance. 

And Allah knows best.