Benny drank 4 over 10 liter of juice.His brother drank as much as him and 1 over 10 more,and his sister drank half as much as him.How much juice did the three children drink?

why clutter things up with all the words? You clearly know how to write fractions.

Benny: 4/10
Brother: 4/10 + 1/10 = 5/10
Sister: (1/2)(4/10) = 2/10
Assuming "him" again refers to Benny. (And it should be "he" anyway)

Now add up the amounts.