Should he let his siblings use his private internet connection?
Question

If a person has internet in his house, and his siblings – both male and female – use the internet, and he does not know what purpose they are using it for, because they are not religiously committed and they may be using it for haraam things, such as chatting between the sexes, or sharing songs, is there any sin on him for that? Does he have to stop them using it?
Praise be to Allah.If you think it most likely that they will use it in haraam ways, it is not permissible for you to allow them to do that, because that comes under the heading of helping in sin, and Allah, may He be exalted, says (interpretation of the meaning):
“Help you one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but do not help one another in sin and transgression. And fear Allah. Verily, Allah is Severe in punishment”
[al-Maa’idah 5:2].
What we advise you to do is put accessibility to the internet solely under your control, so as to avoid any doubt and to have peace of mind and not worry about being responsible for anyone else. It is rare for a person to use the internet without falling into objectionable types of use.
For more information, please see the answer to question no. 112708
And Allah knows best.