I heard from a shaykh (on youtube, Bilal Assad) that in a hadeeth it says that like martyrdom, the people who become hafiz (memorize the entire Qur'an) can intercede for their relatives and family members from hell, and some of my family members are not so religious it seems, and my sister might not even be a Muslim at all but Allahu alem (Allah knows best). I do not wish to make takfeer recklessly lest I become a kaafir. But I love my family, and I want them to be in jannah with me, and though I have hostility between me and some of them (except perhaps my sister), I know in jannah all hostility will be removed.
By Allah's will, there will be a number of intercession of Judgement Day.
The greatest intercession, is al-maqaam al-mahmood, which the earlier and later generations will ask the Prophet (SAW) to intercede for them with their Lord so that He might relieve them of the horrors of the Day of Judgement. This is explained in detail in a Sahih Hadith.
Intercession for those among the monotheists who committed major sins and entered Hell, that they may be brought forth from it.
Anas narrated that the Messenger of allah (s.a.w)said: "My intercession is for the people who committed the major sins in my Ummah."
Jami` at-Tirmidh, Book 37, Hadith 2622
And the intercession from among the believers:
It was narrated that Abu Sa'eed Khudri said:
"The Messenger of Allah (ﷺ) said: 'When Allah has saved the believers from Hell and they are safe, none of you will dispute with his companion more vehemently for some right of his in this world than the believers will dispute with their Lord on behalf of their brothers in faith who have entered Hell. They will say: " Our Lord! They are our brothers, they used to pray with us, fast with us and perform Hajj with us, and you have admitted them to Hell."
He will say: "Go and bring forth those whom you recognize among them." So they will come to them , and they will recognize them by their faces. The Fire will not consume their faces, although there will be some whom the Fire will seize halfway up their shins, and others whom it will seize up to their ankles. They will bring them forth, and will say. "Our Lord, we have brought forth those whom You commanded us to bring forth." Then He will say: "Bring forth those who have a Dinar's weight of faith in their hearts, then those who have half a Dinar's weight in their hearts, then those who have a mustard-seed's weight." Abu Sa'eed said. :"He who does not believe this, let him recite, 'Surely, Allah wrongs not even of the weight of an atom (or a small ant), but is there is any good (done), He doubles it, and gives from Him a great reward.'"
Sunan Ibn Majah , Book 1, Hadith 63
Thus inshallah those who enter paradise will be given a chance by Allah to take their loved ones (who didn't do shirk) out of Hell. But it is more important that we first do good deeds and seek our paradise. Only through it can we hope for Allah's Mercy and Benevolence on Judgement Day.
Regarding the question in the title: No. In the Quran we are told of Abraham and Noah not being able to intercede for their disbelieving father and son respectively. A Prophet's rank is greater than a Hafiz and even a Martyr, and if even a Prophet may not intercede for his family, so how can someone of lower virtue be able to do so.
ما كان للنبي والذين آمنوا أن يستغفروا للمشركين ولو كانوا أولي قربى من بعد ما تبين لهم أنهم أصحاب الجحيم
It is not for the Prophet and those who have believed to ask forgiveness for the polytheists, even if they were relatives, after it has become clear to them that they are companions of Hellfire.
فما تنفعهم شفاعة الشافعين
So there will not benefit them the intercession of [any] intercessors.
فما لنا من شافعين
So now we have no intercessors
With that said you should not speculate about the faith of someone who identifies as a Muslim. A sinner is not a disbeliever.
ثم يقول الله تعالى أخرجوا من كان فى قلبه مثقال حبة من خردل من إيمان
Allah will order those who have had faith equal to the weight of a grain of mustard seed to be taken out from Hell.