Bismillah ArRahman ArRaheem
Wa-Alaykum As Salaam
Your question seems to have changed from "Inheritance" as per the subject to that of "What is my sin?" We'll assume that the question you intended was the one in the content of the email.
Of course, in how you have described the matter, you have not sinned at all, but rather, have tried your best to look after you family and parents. It seems, according to your description, that your deceased brothers wife is trying to maximize her financial return even if she does not have any legal claim over it given that her husband (your brother) has taken the funds with out permission to satisfy his own needs.
A test is sometimes use in order to determine your level of faith, and not necessarily to punish you for a sin.
أَحَسِبَ النَّاسُ أَن يُتْرَكُوا أَن يَقُولُوا آمَنَّا وَهُمْ لَا يُفْتَنُونَ
Do men imagine that they will be left (at ease) because they say, We believe, and will not be tested with affliction? 29:2
وَلَنَبْلُوَنَّكُم بِشَيْءٍ مِّنَ الْخَوْفِ وَالْجُوعِ وَنَقْصٍ مِّنَ الْأَمْوَالِ وَالْأَنفُسِ وَالثَّمَرَاتِ ۗ وَبَشِّرِ الصَّابِرِينَ
الَّذِينَ إِذَا أَصَابَتْهُم مُّصِيبَةٌ قَالُوا إِنَّا لِلَّـهِ وَإِنَّا إِلَيْهِ رَاجِعُونَ
And surely We shall try you with something of fear and hunger, and loss of wealth and lives and crops; but give glad tidings to the steadfast, 2:155
Who say, when a misfortune striketh them: Lo! we are Allah's and lo! unto Him we are returning. 2:156