i don't really think it's ungrateful of you - you're not obligated to take 2nd hand clothes, and it sounds like you've tried to make the point very politely. sometimes it's easy to forget that we are entitled to say no!

i agree with maruscke that maybe things have got a little confused along the way - your bro's GF probably thought it was a friendly gesture to his family (which it is & i can see you appreciate that side of it), your bro prob thought that he's keeping the peace with his GF by passing on the clothes & he prob doesn't realise that storing clothes can take a lot of room!

maybe when you get them & have checked through them, you could ring her to say thanks? you can always tell her then they're too big or whatever but you really appreciate the gesture?