Why do you assume it is done to bring attention on her? That presupposes that she deliberately got pregnant and planned it all around her brother's wedding.
What a ridculous thing to say. We can assume it is to bring attention to her because that is the exact reason she wants to tell them 'they will be over the moon', she wants to celebrate with them! Which is absolutely fine, but 'celebrating' a pregnancy before 12 weeks is not the norm at all, for good reason, so her wanting to do it now, rather than 2 weeks after the wedding and after the 12 week scan is pretty obvious she wants to use their wedding as a way to jointly celebrate, but imagine if she tells the family now and has a miscarriage sometime before the wedding.. hardly a celebratory occasion then, there is a reason people wait for 12 weeks before 'celebrating'.
If she was 12 weeks along now it would be different entirely, so I'm not saying she is doing it maliciously, the timing is bad that's all, but it certainly lacks respect for her brother and wife to announce it early to family without talking to them first. Weddings, even small ones, are a huge build up, lots of organising, stress and anxiety, and the day should be about the people celebrating, not someone else sharing big news (too early) to hijack their celebration.
That is why I said she should tell her brother first, because he may be overjoyed and want her to tell people (I would be if it was me, although I'd advise caution), or he may prefer that after all the effort they've put in to the wedding, and the uncertainty about the stage of the pregnancy that she tells people after or at the end of the night, or even let her do an announcement on the day.. it's his big life event so he should really be given the choice.