First of all, thanks to everyone who left comments on my decision about whether to be a bridesmaid in my brother's wedding. They all definitely helped me in making my decision. I don't think the issue is simple or black-and-white. I needed to look at multiple variables and for circumstances which are likely to arise, before making a decision.
Some of these I thought of myself, and others were brought to my attention through comments on my previous post. One major and upfront concern was being able to dress appropriately--a problem that the bride herself has helped to alleviate. There are many options for me to be able to meet my Islamic obligations of covering while still fitting into her bridal party, by coordinating the color if not the style of the dress.
Another concern for me was being sure of understanding from (her) family so nobody would be harassing the bride for having a muslimah in her wedding. On the other hand, the major audience objections are likely to come from my own family, and some family members who persist in their objections to my wearing hijab. But I share the opinion of the bride, that this is her day and really she should be able to do what she likes.
Some concerns that Jamilah brought up also required some thought from me. The issue of drinking is quite serious, I'm not sure if everyone realizes it, but it's quite clear that not just drinking is haraam, but even sitting at a table where alcohol is served is haraam. That's something important to consider--however, it would affect me by my presence at the reception, and not be impacted by whether I decide to be a bridesmaid. And whether I am or not, I'm sure I may still evade the situation like I do at family events--by sitting at the "kids' table." Since the reception will not be formal (though there will almost definitely be alcohol served), I think I can remove myself from that uncomfortable situation.
Another issue is pairing up at the end of the ceremony--something I didn't even think of. But I'm sure it won't be a problem. Firstly because the ceremony is not going to be especially formal, I don't think something so simple (avoiding looping arms with the groomsman) will be a problem at all. Plus, I know who the groomsmen will be, and I don't think it will be difficult to arrange.
There is also the matter of music and dancing at the reception. I hate dancing in public. However, it's something I can't even seem to avoid at Muslim weddings, where I feel uncomfortably pressured to dance in the first place. On the other hand, I've always found non-Muslims in general to be more understanding when someone refuses to dance. This said, I certainly don't intend to dance at all, but I don't think there will be any problems with it, and again it won't be impacted by whether or not I am a bridesmaid. (I.e., no special bridal party dances or anything.)
Reading the question answered on islamqa.com, the question being similar to my own situation, I think the greatest concern was that participation in a non-Muslim ceremony, specifically a Christian one. But for me I don't think this will be the case. Though the ceremony is non-Muslim, it's not specifically Christian, involves no rites or sacraments in which I must take part. It doesn't require that I invalidate my own faith in anyway, nor is there going to be an overt display of any faith at all that my presence would implicitly condone.
On the other hand, being a bridesmaid provides me with a unique opportunity to actually foster familial ties, especially with my two sisters, who are also going to be bridesmaids, and my nieces, who will be flower girls. I don't think it will require that I compromise on my faith or ideals, but is in fact a good opportunity to strengthen my relationship with my family. Whereas my refusal (to be a bridesmaid) would hurt that relationship, and my absence (from the wedding, if I didn't go) might cut me off entirely.
So ultimately (or at least so far) I have decided that yes, God willing, I will be a bridesmaid in my brother's wedding, while taking precautions to avoid situations that might compromise my faith.