@dadpad,
You say that your mom took this life insurance policy out on you and your two brothers - was your sister not also one of the insured? If not, do you know why not?
Is your sister married? Could it be possible that your mother named your sister as a co-beneficiary in an effort to help her be financially cared for after your mom's death?
The reason I ask is because if your sister is not married, she may not be the beneficiary of any life insurance policy other than this one, and maybe your mother looked at this policy as a way to help insure your sister's financial security and solvency as she ages.
You say that you have your own life insurance, and you've probably named someone other than your sister as the beneficiary - same with your other brothers. Maybe this was an effort on your mom's part to take care of your sister. Do you see what I mean?
For example, I have four nondisabled and married siblings and had one learning disabled, never-married sibling (he died). But if he'd lived until both of my parents had died, I know that my parents had made that brother the sole beneficiary of several insurance policies aside and apart from the rest of their estate- in an effort to make sure he was cared for into his old age, if it had turned out that was needed. We all had the ability to work and care for ourselves, as well as spouses who had named us as beneficiaries - whereas he didn't- so he needed to be specially looked after and cared for financially in the event of their deaths.
Could your mom have set something like that up for your sister?
If that isn't the case - it does seem a little weird for a sister to have life insurance policies on her brothers- yeah- I can't picture my brother having a life insurance policy on me- so I get what you're saying.