I think your expectations are a bit unrealistic. She is 12. How many 12 year olds are really OK for their mother to disappear on holiday with their boyfriend and his family? And would she really either want to go herself or feel happy for you to go away with them? But if you have just acted as if this all fine and there can't possibly be a problem then she can't express her true feelings which might be anger, fear, resentment, jealousy, and instead she has had to make up a fantasy about why she feels that way.
Also you are expecting a new baby. She may be looking on the bright side and being pleased in a way about having a new brother but really (if she is just a normal 12 year old girl!) she will also be having a lot of other feelings as well - resentment, jealousy of the new baby, fear of being replaced, fear of losing you to your boyfriend's family.
You've said it's OK for your DD to feel upset about death, family illness, school problems. But other things - family changes - that are genuinely upsetting for many young teens, you haven't mentioned those. And if her mother doesn't seem to understand her mixed feelings about the new family then she wont expect anyone else (like her friends) to understand her feelings either, and instead she has made up a story about care and geting hit and so on, so that her friends will sympathise with her feelings.
So what to do? The stories are not really a big problem, they are so obviously made up. The problem is that she is unhappy about what you are doing and she can't tell you how she feels. So have you said to her that it's OK to feel upset about you going away on holiday with your boyfriend? Or that it's OK for her to feel anxious or resentful about the new baby? Have you reassured her that you will still have time for her, still be there for her? Those things will help to make her feel it's not necessary to make up stories to get sympathy.