Everything CIC told you is correct. You will have to travel to London to get a PR Travel Document in order to travel to the US.
There is two possible ways around this:
- if your mother is seriously ill, buy your tickets, see if you can get something from the hospital about her condition (on official letterhead) and submit it to CIC asking for urgent processing;
- if you can change your flight to travel to the United States (with an ESTA) instead of Canada, fly to the nearest US international airport to your home in Canada, and have someone come pick you up (you must travel by car) as you will not need your PR Card at the border.