Just uploads, hmmm. Long shot, but my ISP once throttled uploads only, and it took a TCP dump analysis to discover the problem.

I may have missed it, but did you remove the router and connect directly to the brother?

Yes I did.

I gave up in the meantime and simply installed an FTP server they can connect to without a problem and get fast downloads.

After 4 lost days of trying to find a solution, I have one simple key finding for all users who want at times to exchange their files with their friends only and use Limewire to do that.

You can connect directly to them yet, you do not need to be connected to the network!!!
Should you not have an issue with any router and be able to vice-versa download, you can thus give your friend full attention and priority.

For all users who do have routers and LANs with several concurrent computers like me, and can not achieve quick downloads from and to friends, I recommend to get the serv-u ftp server which is simple, affordable and allows you to trial it for 30 days with the full corporate version. Then, the simple home edition should do for a handful of friends who want to get your movies. $40 and 5 minutes later you have a full-fledged secure FTP site, they can access for instance with the Total Commander (the best free tool opn the web, making Windows Explorer redundant, with many other top built-in function such as top notch packing/unpacking, syncing of 2 folders accross a network or on the same drive for efficient file backups and much more - you can find it under http://www.ghisler.com/whatsnew.htm)

I am out of here...