Jack Flash Jr wrote
Cape winter is not as bad as it's reputation, that said, you might hit a rainy streak. July might still be Indian Summer (I kid you not). It's not as cold as JHB and there is still plenty to do of course.
Jack Flash, I think you've been living here for too long... ? When it's been raining (read: pissing down sideways with 60km/h winds) for 10 days straight it can get pretty grim, however as Mr Flash has said, in between the misery we also have the most sensational sunny, autumnal days which are good enough for hitting the beach! Not nearly as cold as up north, and there are always a multitude of cosy places to go and drink port, eat home-made soup, taste wines etc. Also, die Kaap is beauuuutiful and bonny-green in winter, with waterfalls on every mountain etc, so it's very scenic.
To be honest, if your fandamily are coming from the UK, they've probably experienced worse weather in the middle of their supposed 'summers' so they should find it a breeze. It's only those of us who feel cold when the temp drops below 25 degrees, and who're used to chilling on a beach for 4 months of summer who take strain here in winter! Just tell them to pack a wide variety of clothes (and not to bother with umbrellas) and they should be fine!