Would it be easier to talk to your brother? I'd go that route if it'd be easier.
But regarding what to say, since I've had to have a similar discussion with my sister (who is also difficult to talk to), I'd go the safety route. I can't link you right now but someone can or search these forums for the thread about Josh & why RF is safer. There's also some really good (aka scary) videos on youtube showing what happens to babies's heads/necks in a crash when they are in a FF vs RF seat. Transport Canada also has crash test videos on their site: while I wouldn't necessarily suggest them as a good source there's a few that show a RFer right next to a FFer so its very easy to see how much safer it is for tiny necks in a crash.
You could also remind them that not only is it illegal for a child under 1 year to FF (I know they'll say she's so close its not worth worrying about), but also that Transport Canada and the Canadian Pediatric Society recommend RF beyond 1 year, to 2 if possible, or the limits of their seat. If you think appealing to their willingness to spend big might work, recommend they check out the Radian XT: its not the most expensive seat but its up there, and it RF to 45 lbs, with a tall shell & harness height. Also, if they say they're worried about her legs, say "broken leg = cast it, broken neck = casket."
If after all of that, they don't want to hear you, cut your losses. You can only educate those willing to be educated and you need to try to forget about those you can't. I've had to say my piece and leave it at that with my sister, who's been in 3 car crashes since she was pg with my nephew, turned him FF on his 1st birthday, has a new baby who rides with a bundle-me and a loose harness too. If I ever have to have her kids in a vehicle I am driving, they will be RF if their seats allow it, with properly adjusted harnesses and no 'accessories'. That's all I can do.
Good luck,
Melissa