On the other hand family can be more forgiving (well, some are). That being said, kind of agree that a 1st time brisket attempt for a big family dinner is a bit on the bold side. Best of luck.
Remove the hard fat, leave a 1/4" fat on, highly recommend going true on seasoning, using 50-50 salt & black pepper that reasonably covers the brisket. Just not so heavy that the meat can't be seen. Weight all the trimmings so you can know how much weight will actually be smoked. A number of variables will affect the timing. Weight, smoker temp and the piece of meat itself. Know your grate temp, hood gauges are notoriously inaccurate. Lately I've been smoking at 270º and with a trimmed packer of around 14# and they been getting done roughly around the 45 min/lb mark. YMMV