1) Is there any way to block his pension?
2) Is there any way to get a portion of his pension in our brother's Name
NO is the answer for both the above questions.
Law is common for all.
There are no considerations for sentimental issues in law.
Since your sister was married prior to her death, the succession law operates and as per that succession law, the legal heir or successor bing her husband, shall be eligible to succeed all her estates.
Hope you understood the position of law.
What is the reason you think you can block the pension?
Well, if you block the pension, nobody can avail the benefits, it will go back to the government itself.
Despite this, if you are adamant to block his pension then you may gather the evidence of his second marriage with another woman and send a petition to the employer of your deceased sister.