I am looking for the name of a movie I saw in the 80ies as a kid and loved. I vaguely remember a sister (brown hair) and her younger brother being on a mission/ good vs evil. One part is where they are hiding out in a museum at night and gathered lots of pennies out of a fountain.
Thanks for any help!

This sounds like the movie version of From the Mixed Up Files of Mrs. Basil E. Frankweiler.

The movie version is "The Hideaways"

1
Go to the Internet Movie Database, The Internet Movie Database (IMDb). It contains a large database covering movies and TV shows of all varieties, and its search engine facilitates any number of approaches to deciphering a movie's title.

2
Click on the "Characters" option in the search window, then write the name of the character. This will take you to the character's home page, which ideally will have the name of the film itself. If the character appeared in more than one film (Sherlock Holmes, for example), there may be more than one movie listed. In some cases, there won't be a listing for the character, in which case you should return to the IMDB home page and start again.

3
Click on the "Names" option in the search window, then type in the name of the actor or director. You'll be sent to a page containing a list of all his work. Scroll through it until you find the name of the film you're looking for. This may take a little more work, but if you pinpoint the film name, you can be more assured that it's the right one. If you can't find it, go back to the IMDB home page.

4
Click on the "Plots" option in the search window, then type in key phrases connected to the plot. Examples of key phrases include "spy movie," "romance" or "explosions." Try to be as detailed as you can and include as many key phrases as you can. The engine will then take you to a list of plot summaries that include those key phrases. Sift through them until you find the one that matches your movie.

Thank you so much,@ kcomissiong - indeed :)
Your expertise is much appreciated.
Take care!