In the first sentence, the particle after へや should be で instead of に since the action does not include movement (they're talking in the room, not toward the room). Also, the reply should be ははが(はなしています), just like in the question.
The location of the time indication (such as 今) doesn't really matter as long as it's before the verb, so the second sentence would be grammatically correct. However, since you're talking about a younger sibling, the さん is not needed - this is only for family members who are older than you like おねえさん and おかあさん. And of course, if you're talking about your family members to someone outside of your family, you never use an honorific (like はは).