Who saw Ralph when the island was on fire and didn't say anything? Why did they not alert jack?

I believe you're talking about when Ralph is in hiding and he sees Sam and Eric (they warn him Jack will be sending out the whole tribe to find him the next day). The twins don't alert Jack because they care about Ralph, but they won't join up with Ralph again because they know how it would end up, and they're afraid....... with good reason, I might add.