I can see this completely from both sides.
I have 7 children and can say that with every single new baby came a new set of hands to 'maul' him/her by way of the older children before them.
The constant touching is normal. However, I know the irritation it produces even with full brothers and sisters - but it is for the most part irrational. My mother (gentle and non-violent )summed up the feeling perfectly once when DC3 was fiddling with DC4's clothing; 'Yes, I can't put my finger on whats so annoying about it, but you want to kick them to high heaven when they won't stop touching the baby'.
She had hit the nail on the head. DC3 wasn't doing anything wrong or anything to threaten the safety of the baby and yet I felt like exploding every time she reached out to touch him. And I honestly didn't know why. I have tried to analyse it, but I can't see why. Loving them all equally (being all my children) though, forced me to have sympathy for the villain and after snapping a few times, I made a huge effort to be more patient in the future.
However, that irritation feeling faded so much more once the babies reached 9 months, it's almost a relief now when someone picks up a crying baby for me. And as another poster said, a 9 month old is nowhere near as vulnerable.
But at the same time as feeling sympathy for you, I feel so sorry for this little girl who wants to touch the babies out of instinct and whose Daddy now has some new babies in his life, when she was the 'baby' before.
My current baby is 6 months old and is the first of the children with a different father. My new husband has an amazing relationship with all the other children but he does fret sometimes when Monty is picked up by the 10 or 6 year olds, whereas I, like your husband, am far more relaxed with it. I think experience has told me that none of mine came to harm in this way before.
I wonder if you can try your hardest to imagine one of your children in your DSD's role. How would you like a stepmum to react to your child? That may help you a little because I know I snapped at mine many a time, but would be mortified if anyone else had spoken to them sharply like that .
It will get better as they get older and hopefully she will become a little helper to you.
And if you have any more, you'll see your twins turn into the mauling monsters too