ANSWER:

Your sister is sinful, she can let her daughter study online, and in this way she will not be forced to attend  such haram events. This is a critical age and she is setting the foundation to her daughter whether to be a real Muslim or not. 

My sister sends her 8 year old to a non-Muslim school as there are no Islamic schools in her town. The school celebrates non-Muslim festivals, has regular singing, glorifies witchcraft and serves non-halal meat. My sister thinks it’s not so serious because her daughter is still young. Is this correct and at what daughter’s age should she relocate to fix this problem?