Where can I find help to clear my mother's apartment in Washington, DC?

Do you mean that mom's house looks like an episode from Hoarders, and you want to dispose of items? I'd call the local Area Agency on Aging and find out if there is a cleanup agency they'd recommend. Or the exterminator might have a suggestion.

All clothing that is being kept must be washed and dried at high heat, and then sealed in contractor bags until the exterminations process (which requires 3 visits, a week apart) is complete.

Do your research on removing the items from the home. It's very risky, based on all that I have seen over the years. Regular stuff doesn't kill them anymore.

Often, I've seen that killing bedbugs is extremely challenging. And sometimes the only thing to get them, eggs and all, is to treat the entire house with very high heat. That is expensive. I'd be very hesitant to remove belongings from the house, even after it is treated, because treatment of them is very iffy and they could infest the new location. I bet you will get some stories of personal experiences about this. I have seen quite a few on tv shows that tell how families have been driven out of their homes due to bedbugs.

Do not move the infested items. You will be taking the bedbugs with you. Ask the exterminator.