My NT son (5) has gone through a phase for about a year... That got worse when he started attending my autistic son's (6) developmental preschool (blended program with NT and NA children). My 5 year old started mimicing other children's behaviors as well- so good in fact he could have gotten an academy award for his performance! Young children mimic others as a way to better understand them, to make sense of the world around them, to relate, and of course to seek attention- sibling gets attention for their behavior, so shall they! (An infant may do it simply out of mimicing the world around them). My son's preschool teacher and I discussed the issue- because you can't exactly tell a kid they can't act like that because it's WRONG, because that could send a bad message about NA/ developmentally disabled children. It was tricky but our main approach was to remind him to use HIS words, HIS voice, or remind him his brother/ other children were like that because they learn differently than him- and he didn't need to copy their behaviors. We also talk to him about how some of their sibling's behaviors aren't desirable, because they're not off the hook just because they have a developmental delay. They are trying to learn socially appropriate behaviors too, and their sibling needs to help model those behaviors for them- letting them know that can give them a sense of purpose in behaving normally. And sometimes... as you can with any undesirable behavior... You can ignore it, and it will lessen or go away completely. I also feel it is something that won't outlive the preschool/ early years so much. Good luck!