Flyingcloud, I feel your pain. I think they put the pressure on you a lot in France.
I'm French and one one of things I hate about France (and was one the reasons I've been happily living in the uk for almost 9 years) is this obsession with weight. I know they weigh you every 5 min in France when you're pregnant and that you're not supposed to put on more than 10 kg, a bit more than 1 stone...
Tell your BIL to F off (let me know if you want colourful variants for this). It's because of people like him that women become anorexic and p*sses me off like you can't imagine.
As someone who suffered from anorexia for 5 years, I was a bit worried that I'd freak out during pregnancy but I haven't. Don't get me wrong I still have fat days when I feel like a baby hippo but I really went with the flow. I usually eat very well but almost lived on haribos around xmas and usually 1 day a month I turn into an eating machine (proper hunger not just cravings).
I never weigh myself. I must have put on some weight (my stomach is huge) but probably not that much since I can still wear my pre-pregnancy jeans (with a boob tube to hold them up since I can't close them). I reckon that if you listen to your body, eat when hungry, not too much crap and a few treats, you'll be fine. I have no intention to diet after. I'm going to breastfeed, start exercising again when I feel I can, carry on eating well and not weighing myself. I reckon that I'll go back to my normal size (under 9 stones) naturally.
Good luck!