First of all, mazel tov on your brother's wedding! I hope it was a wonderful occasion for your family and that the sheva brachot meals continue the celebration. (Sheva brachot refers both to the seven traditional wedding blessings as well as meals honoring the couple for the week following a wedding where the same blessings are recited that were said during the ceremony.)
Presumably, you've already met your brother's in-laws, at least at the wedding if not before. So if they planned to ask intrusive questions, they've likely already done so, either to you directly or to your brother and new sister-in-law. I see no reason to expect to be ambushed during the sheva brachot meal, since that should be all about celebrating the new couple and not about interrogating the guests.
However, weddings do make people think about other weddings, so it's possible that your relatives, regardless of their religious affiliation, would take the event as an opportunity to ask you about your own plans. Any time you get such questions, regardless of the context, feel free to say, confidently and politely, "When there's news, you're sure to hear it," or, "I haven't found someone I want to spend the rest of my life with," or, "I'm very happy the way things are," or even, "I prefer not to talk about my personal life." Your age, your religion, or the age/religion of the people talking to you doesn't change the basic rules of polite conversation.
As for the atheism question, again, it's hard for me to imagine how this would come up unless you supply the information, in which case I would advise you not to bring up your own beliefs. If you're asked directly about your observance, again, confidently and politely, say, "I don't like to label the way I practice Judaism," or, "I belong to a Reform (or Conservative, or Humanist, or whatever) synagogue," or, "I'm Jewish, but I'm not observant in the same way you that you would define it."
Since it's an Orthodox setting, there's not even the possibility that you would be asked to recite one of the blessings, so you don't have to worry about having to politely decline that, if such a thing would be outside your comfort zone.
Before you go, though, find out the expected dress code from your brother. Depending on where the family is situated along the Orthodox spectrum, you could end up being the only woman wearing pants. So if that's something you want to avoid, either for your comfort or that of your brother's in-laws, consider wearing a skirt and a shirt that covers your shoulders. It's not hiding who you are to comform to the norms of your hosts.