My grandfather was not Muslim and had sons with women he was informally (not religiously) married to. Are my uncles born from these women considered to be my Mahram or must I cover in front of them? This is a very hard issue because my family is very close and it hurts me to think that I may have no relation to them.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

They are not Mahram to you since they are not legitimate sons of your grandfather. Because the Mahram is any man who is eternally forbidden from marrying a woman because of blood ties or breast-feeding or because of marriage relationship.

Adultery or fornication are illegal ways, so they do not cause legal lineage or marriage relationship. So you must cover yourself from all those persons because they are not your Mahaarim (plural of Mahram). However, even though they are not Mahram to you, you should not marry them, since they are from the semen of your grandfather. This is the opinion of the majority of the Muslim scholars. Your goal should be the pleasure of Allah. Allah Says (what means): {And whoever fears Allah - He will make for him of his matter ease.} [Quran 65:4]

If you suspect any harm from them and you are married, then you have the right to demand your husband to provide you with a separate residence. You should not stay with them in privacy without a Mahram of yours. If you are not married, try your best not to be in privacy with them and observe Hijaab in their presence till Allah blesses you with a husband.

We ask Allah to bless you and bestow his grace upon you.

Allah knows best.