When filling I-130 for a sibling who is currently in US and already overstayed, do I write that information on the form?

Chances are that no one will go after them but it will be about 10-12 years before a visa would become available and even then, as you know, they would not be eligible for a green card under the current immigration laws, if they have overstayed.

You have to accurately complete the Form I-130 petition or it may be denied or if approved, may be revoked if DHS later finds that it should not in fact have been approved. F-4 petitions takes about 12 years to approve and if things are not done properly, the 12 year wait could be for nothing. So, you must disclose all necessary information including the fact of his physical presence in the US. Absent extraordinary circumstances like criminal history or prior removal order, DHS is unlikely to go after your brother because you filed his I-130 petition. But you must also know that an I-130 petition gives no status and for any illegal alien, risk of deportation is always present. Finally, until the immigration reform becomes law, you are bound to work with the law as it exists today.

You should answer every question on the Petition honestly. There have been instances where, as a result of Petitions filed with the California Service Center, they have gone out and picked up the beneficiary, but it has been a few years since I have heard of that occurring.

DHS generally does not "go after" beneficiaries in I130s unless they pose a threat or they come upon them or could have obtained their information in some other fashion, such as being turned over to ICE custody after a criminal arrest or conviction.