My brother passed 6 days before my mom. Mom was beneficiary on his pension, but sister-in-law had it changed to her. Is that correct?

Rosalind, are you sure you are using the correct terms? By law, the spouse MUST be the beneficiary on a worker's pension unless the spouse signs away their rights. But, many employers offer group life insurance. Anyone can be the beneficiary of the group life policy.
Here is one scenario: Mom was the beneficiary of the group life. (Often happens as worker was single when he signed up for benefits and never changed it) SIL may have been reviewing things if your brother was sick for a while and had it changed (with or without his knowledge) .
If it is the group life you are talking about, you can call the insurer and ask for documentation on the change. Actually the vast majority of group life policies are paid to the spouse. After all, she incurs the financial loss when he passes and that is the intent of life insurance.
Let us know when you clarify.

This sounds wrong and should be looked into by the police.

Dear Rosalind,

My deepest condolences and sympathies. I'm very sorry for your loss.

If you suspect fraud I would check with an attorney.

Not that I have ever seen. My husband worked for GM my Dad DuPont. Upon retirement they sign paperwork for survivor benefits. As far as I know, only a spouse can be named but ur Mom must have been OKd. The pensioner gets less money monthly when this done. Since Mom died, not sure how wife got it when looks like brother didn't want her to have it. She must of had to produce death certificates and a marriage certificate for proof. I do know divorced woman who get partial pension.