please see the attachment and answer
Answers
The better suggestion I would give u is to message just as a friend him when he comes online but there might me some chances that his / her parents are checking their phone so they might not message you.
If they reply you back then talk to them in a polite way don't be so serious about why they are not texting you ask them if there is any problem in the house or with you. Remember one thing always speak in polite manner and don't provoke them because it may in return hurt you more than anything .
If they say that they have problem with you then block the contact and try to distract yourself from them as they may cause severe issues in you mental health.
That's it what I want to say you as a sister because I have also faced such problem from my side
I hope you get a good reply from their side
#Stay safe #Stay happy and always keep smiling