Died before benifits approved

You will want to confirm with the SSA, but retroactive benefits seem unlikely.

The main problem is that there is a waiting period before disability benefits become payable. For example, see this excerpt :

If deemed disabled, you will have to wait six months to get a check. You should apply for disability as soon as you become disabled. It can take a long time to process an application for disability benefits (three to five months).

If your application is approved, your first Social Security disability benefits will be paid for the sixth full month after the date your disability began.

For example, if the state agency decides your disability began on January 15, your first disability benefit will be paid for the month of July. However, Social Security benefits are paid in the month following the month for which they are due, so you will receive your July benefit in August.

http://ssa-custhelp.ssa.gov/app/answers/detail/a_id/455

However, if the mother was a dependent of your brother-in-law, and she is age 62 or older, the mother may be eligible for survivor benefits. See here for details :

http://www.ssa.gov/ww&os2.htm