The rule
In a lot of family words the umlaut is the plural — there is no -s, no -en, just a change in the vowel. If you miss it, you have said the wrong number of people.
Singular → Plural
| der Bruder | die Brüder |
|---|---|
| die Mutter | die Mütter |
| der Vater | die Väter |
| die Schwester | die Schwestern |
| das Kind | die Kinder |
ü is not u. Round your lips as if for „oo", then say „ee" without moving them — that is the sound, and it is the only thing separating one brother from several.