M.P.
How old is your sister? Did she do or say anythng to which your daughter was responding? There is always two sides to the story. I agree that your daughter should not have slapped your sister. Also that she shouldn't have asked your sister if she's pregnant. Remember, however, that at 8 she may not have realized this was a rude question and was just asking a legitimate question. If she asked it because she wanted to irritate your sister then she was wrong there too.
I suggest that sitting for the rest of the evening is sufficient discipline. The next step is to work on understanding why your daughter is acting this way. I suggest reading the book, How to Talk so your Child will Listen and How to Listen so she Will Talk. You need to open a dialogue with your daughter so that you can focus on teaching her instead of punishing her.
Once your daughter understands that she was wrong she needs to apologize to your sister. You can not make her understand. You have to be an sympathetic listener and find out the entire story from your daughter's viewpoint. I know that is not always easy to do, especially when this happens in a family get together with emotions running high.
Listening does not mean that you agree with what she said. It means hearing how she feels behind what she says and helping her learn how to deal with the feelings in an appropriate way. It means asking her questions that lead to an appropriate answer which in this situation is what would be a better way for her to have acted.
Perhaps your daughter needs more time spent with you doing fun things during which the two of you can talk about what is going on in her life, how she feels about things, and what are appropriate ways of behaving. Perhaps do some out right teaching in a respectful way. Talk about how she might feel if she was a fat girl and someone asked her if she's pregnant. If your sister is young and your daughter feels put down by her talk about what your daughter can do without being rude. Talk about life in general and how to treat other people.
Most important is to build up your daughter's self-esteem. Spend time with her letting her know that you love her and love spending time with her. Praise her every chance you get. My daughter told me that she felt that I didn't praise her enough and she wasn't always sure that I approved of her as being herself.
If being at a family get together is difficult for your daughter,find out why and together brain storm ways that you and she can make it easier. I'm thinking that their is possibly tension with in the family because your sister wanted to leave without telling you about the incident. Not talking about this sort of thing creates tensions within a family. We all need to feel comfortable knowing that we know what is going on within the group. We all need to work out our relationships with each other within the group. An 11 year old child should be be the cause of another member of the group leaving especially if the other member is an adult. If your sister is a teen, then I feel even strongly that there is more to the story.
I grew up in a close family. If such an incident had happened, the aunt would have told the mother who would have quietly taken the 11yo into another room and dealt with it. When the mother and child came out the child would have apologized to the aunt even if she still felt that she had been right to ask the question and slap the aunt. Everyone would have then let the matter drop and continued to be gracious to the child. The child would be allowed to sulk but not in the presence of everyone else. Eventually, the child would have cooled off, regained her composure and rejoined the family. Even in the presence of anger, we would be family and would show love for each other.
Is it possible that this incident is a symptom of something much deeper going on between your sister and your daughter or with other members of the family? I may be hearing wrong but it sounds possible that your daughter is the scape goat. She was definitely wrong. She should always keep her hands to herself. Why did your sister want to leave without talking with you?
I'm not finding the right words to express myself. Although I do think that your daughter needs to be disciplined, I also think that someone needs to listen to her with a sympathetic ear.
After writing this post I went back to look at your other posts. Your daughter is angry and has every right to be angry. She needs help in the form of counseling. Having been molested and then having to go thru three years of court involvement would make anyone unable to function well. Please cut her some slack. She is being rude and not keeping her hands to herself because she is feeling a great deal of pain that is coming out in anger. Sympathize with her as well as provide guidance with appropriate consequences for misbehavior! Please try to understand how she feels! Please get her into counseling! The District Attorney or the police officers who worked on this case can tell you how to get her some help.