So as per topic, what should i do? He is standard 5 iinm. I dont care if he watches straight porn since i think most of us did the same at that age 

Found out by checking the browser history since he always use the computer in my room whenever im not around to play games and watch youtube. But one particular day he locked the door whereas normally he wont even close the door properly. Since then i always check the history and yeah he did watch it on other days as well.

I want to tell my sister about it so that something can be done to correct him before its too late, but at the same time i dont want my sister and my bro in law to be very disappointed that their son is gay. And also im afraid he will do something with my kids if i expose him, although i can ask my sister to say she found it out herself by checking the browser history

Another possibility would be he was simply curious on how to masturbate 

His google search was always “boy cum” 

I think u r confused. so i'll answer these questions. Btw, the uncle = me aka TS.

This is at my parents house. Of course i have my room at this house. I stay at my parents house. But my room is still there for whenever i wanna go back. So my father bought a gaming PC and put it in my room, since my PC is at another house. So my nephew and niece always use my room to play games, watch youtube, etc. They stay at my parents house together with my sister and bro in law.

My nephew has his own ipad. I dont know if he watches it on the ipad as well. Not gonna check

He only knows how to play games, youtube. He doesn;t know about VPN, incognito mode, etc

What porn u watch until need to use VPN? I simply use some free dns enough to bypass. And i have tried without dns or VPN, there are some sites that can still be opened. You can try it yourself

I tried opened some of his history, the ones i remember was guys jerking off, and one more is guy pounding another guy's ass