I cared for a little boy with "classic" autism for years - he was 4 years older than my DS, and they were essentially raised as brothers, DFS joined our family when DS was 4 months old.
As DS developed, it was "off". I had a voice in my head that kept whispering to me that my PFB had some sort of developmental issues, but there was a louder voice that hollered "he has been around a child who has obvious challenges, and he has learned the behaviours - get over yourself".
DS was admitted to a SN Nursery, was followed through the Early Intervention program locally, required therapeutic intervention, all of the professionals who worked with him acknowledged these issues, but that voice kept telling me to minimise it all.
However... when DS started in MS school, with NT peers, and DFS was no longer in our care, those behaviours remained.
It took me years to finally tell that voice to sod off, and listen to the whispering instinct before we finally agreed with the Paed that DS was not NT and required assessment. Turns out he has ASD as well - and it is not contagious or learned - it is what it is. And, to be honest, there is an increasing body of evidence that says it runs in families - lending huge support to the "genetic" theories...
DS is a challenging young man - "Gifted/LD" with ASD, but he is really an incredible young man, too. As he has gotten older, he has started learning (SO slowly) to manage his anxieties, and his sensory seeking/avoidance, and his need for structure, and his VERY selective diet. He has found a small but tight group of friends - all of whom are similar to him.
When he was 7, his little sister was born, and when she was 2, she was diagnosed with ASD... and we discovered that autistic features are a trait of the larger disorder that they both have.
What I am saying, is that unless your DD is exposed to NO children aside from her older brother, mimicry is not a likely explanation, unless your DS gets an inordinate amount of attention because of his challenges (and I am not seeing that on your description).
Listen to that voice that says "ask for assessment". Keep diaries, ABC charts, you know the drill, and go to the GP with a good argument for your request.
And... keep expecting things from your child - cleaning up at night, bathing, wearing clothes, eating. It is exceptionally challenging, but you and she will learn what is non-negotiable. If she is getting all the nutrients she needs on her limited diet (perhaps with a vitamin added) then don't make eating a power struggle. Have her help choose her clothes, but if she chooses them, she has to wear them. Cleaning up at night is done - whether she caries on or not, it is done.
And... for what it's worth - you are concerned about this... that categorically makes you NOT a crap parent.