advice needed for 9 yr old feeling overly reasonable for younger siblings

[WellHurrahAndHuzzah]

My dd (9) is overly responsible for her younger siblings. It's like she doesn't trust me to keep them safe. There have never been any incident to suggest this is the case. She can't stop worrying about them when we go out eg worrying about traffic etc. How can I get her to call down and enjoy being a sister to them?

Any advice really appreciated

Not the same thing I know, but I adopted my DDs then age 8 and 2. DD1 had had to care for DD2 when younger.

For a long time I had to keep stressing to DD1 that 'I'm the Mum' and also say things like 'that's for me to say/do'. Actually I still have to do that sometimes (DD1 is 15) e.g. when she tells DD2 it's bedtime and stuff like that.
I also made sure I didn't ask her to keep an eye on sister in the house or in the park, to keep up the fact that we were the parents.