It depends on how old the young girl. If she is under six she will probably regress and start acting younger than she is to try to gain attention from her parents - this happens quite a lot in children with more favoured siblings so it wouldn't be unusual if a five or six year old started wetting themselves regularly. However if she is between six and eleven I think that maybe the young girl will resent towards the new child, if this is not stopped it could lead to a very dangerous situation in which the young girl is convinced she must hurt the new baby. If this child is over eleven I think she would become reclusive and unresponsive, maybe she would start behaving badly at school (this could also happen with the child 6-10) or begin arguing with her friends and pushing everyone away until she is left alone - which can be a very scary place for anyone of any age.
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