Ask Mr. Smarty Plants

Here's what we were able to find out about Platanus occidentalis (American sycamore). It is monoecious, meaning it has both male and female flowers on the same tree, so another tree of the same species does not have to be present in order to produce seeds. The fruit is a ball composed of many closely packed, long, narrow  fruits that ripen by September or October and often remain on the tree over Winter, breaking up or falling off the tree the following Spring. A plantation or open-grown sycamore begins to bear seeds in 6 or 7 years. Natural stands of the tree begin to produce an appreciable number of seeds at about 25 years. The tree usually  bears a good seed crop every one or two years, with optimum production between 50 and 200 years of age.

Possible conclusions:

1. Either the tree in your condo yard is not a sycamore or your sister's is not.

2. Your tree is a natural growth tree and will not begin to bear fruit and seeds until it is about 25 years old, while your sister's may be a plantation-grown tree. It would still have to be a little older than she thinks it is to be bearing fruit, however.

We are going to provide you pictures of the distinctive leaves and bark for comparison with both trees. If you do not believe your tree (or your sister's) is a sycamore, go to the Mr. Smarty Plants page on Plant Identification, and follow the instructions for sending us pictures and description, and we will try to identify it for you. 

Pictures and more information from Virginia Tech VTree ID

Pictures and more information from North Carolina State University Fact Sheet

Pictures from Google Platanus occidentalis