Will her deceased husband be punished because his mother delayed giving his wife her due share of the inheritance?

Taking possession and gaining ownership of the wealth is one of the
conditions of zakaah being obligatory; there is no zakaah on wealth when
possession and ownership thereof is not established, because when
possession and ownership are not established, the owner of the wealth
cannot dispose of it in his own interests, so it is more appropriate that
no one else should be entitled to any rights over it before possession and
ownership are established. 

The scholars of the Standing Committee were asked: 

My
father died – may Allah have mercy on him – and he left behind in his estate
some money that was deposited in bank accounts. Because the division of this
money to the heirs was not completed until after more than two years had
passed since the death of our father for a number of reasons, some of which
were beyond our control and others were due to the fact that some of the
heirs live abroad and were late in appointing someone to attend court, and
it took some time for those banks to decide about division of the estate, no
zakaah has been paid on that money since the death of our father, may Allah
have mercy on him. As a little more than two years have passed since then,
should the zakaah be paid on the shares of all the heirs put together or
should it be paid on each share on its own, and what is the rate of zakaah? 

The committee replied: 

If
the situation is as described, then no zakaah is due on the wealth of your
father for the two years during which the estate was not divided, because it
was not possible for each of the heirs to take possession of his share, for
reasons that were beyond the control of the heirs, and because some of the
heirs were far away and they were scattered, which was a cause of the delay
in the division of the estate being carried out, as you mentioned. So the
share of each heir comes under the ruling on wealth the ownership of which
is not established, and one of the conditions of zakaah being valid is that
possession and ownership should be established. But when each heir has taken
his share of the estate, if one full year passes and it reaches the minimum
threshold (nisaab), then zakaah becomes due at the rate of one quarter of
one tenth (2.5%). End quote. 

Based on that, you do not have to pay zakaah on any inheritance which you
have not been able to take possession of or have it at your disposal, or
know the value thereof, because your ownership is not established and you
are not able to dispose of it in your own interests or in the interests of
anyone else. If you do become able to take possession of it or dispose of
it, then you must pay zakaah on it from the time when you become able to
dispose of it, if it reaches the minimum threshold, when one full year has
passed. 

Secondly: 

Your husband will not be punished for what his mother does of delaying your
inheritance or depriving you of the inheritance to which you are entitled,
so long as he did not leave instructions to that effect or ask his mother to
deprive you of the inheritance, and he did not approve of that during his
lifetime, because he has no control over this wealth after his death. When
he died, it passed to his heirs and it is now the property of his heirs, and
is not his property. So he will not be accountable for what others do with
his wealth. Allah, may He be exalted, says (interpretation of the meaning):
“On no soul does Allah place a burden greater than it can bear. He gets
reward for that (good) which he has earned, and he is punished for that
(evil) which he has earned” [al-Baqarah 2:286]. 

Al-Bukhaari (6442) narrated that Ibn Mas‘ood said: The Prophet (blessings
and peace of Allah be upon him) said: “Which of you loves the wealth of his
heir more than his own wealth?” They said: O Messenger of Allah, there is no
one among us but his own wealth is dearer to him than that of his heir. He
said: “Verily his wealth is that which he has sent on ahead (by spending it
in charity and good causes, so the reward for it is stored up for him), and
the wealth of his heir is that which he has kept back.” 

After his death it is described as his heir’s wealth. 

Al-Haafiz (may Allah have mercy on him) said: 

Everything that the deceased leaves behind becomes the property of the heir.

End quote from Fath al-Baari (11/260) 

Please see also question no. 117209 

And Allah knows best.