My brother passed away but straight talk refuses to unlock the phone. I’m at a loss of what to do. They are almost forcing me to throw away a perfectly almost new iPhone 6. Help

From their website:

1. Straight Talk will unlock phones of current and former Straight Talk customers without charge. Non-former customers may request unlocking, but Straight Talk may charge a reasonable fee.
2. The Straight Talk phone must have been activated on Straight Talk Service for no fewer than 12 months with service plans redeemed in no fewer than 12 months.
3. Customers must possess a Straight Talk phone that is not reported stolen, lost, or associated with fraudulent activity.
4. If a customer in good standing requesting phone unlocking is deployed as military personnel, upon provision of deployment papers, Straight Talk will honor that customer's unlocking request.

If you do not qualify for any of the above, I'd recommend selling the phone to some one who wants to use the Straight Talk service and using those funds to buy yourself another unlocked phone.

What do you mean by unlock? If you mean remove activation lock, Straight Talk can’t do anything about that. Only Apple can help and only if you have a death certificate and a will or some legal document proving that the phone now belongs to you. If you cannot provide that documentation, there is nothing that you can do.

If you're referring to the carrier lock, Straight Talk is under no legal obligation to unlock the device if it doesn't meet their requirements for unlocking.

Activation lock, as noted above, is a different animal and will require you to provide documentation to Apple regarding the disposition of the phone as part of your brother's estate.