I was just wondering how others would deal with this ?

I have had a lad over the past 4 months who has moved here from germany his english is great & he has been chatting away no problems however his little sister has started yesterday & were she is younger mum said she can speak very well in english & i am to incorrange them to talk it

Heres the problems if girl wants something she snaches etc when i speak to her she then starts talking in german to her older brother & they wont tell me what they are saying so i tried asking mum her reply

" my children are not talking nasty they are brother & sister & do that just ignore it & snatch the toy back"
I explained that's not how i should behave so she said do what ever you think ??

Any advice ?

( Boy is 7 yrs Girl is 5 yrs)