The first question, "is he in danger of getting resurrected with" his "kaafir" relatives, shows an awareness that any kaafir (disbeliever) will be destined for hellfire, so the question expresses a desire to avoid the "danger" of such a fate. The idea that a kaafir will be destined for hell in the Hereafter is made clear in the Quran.
From Surat Al-Maaida 5:10:
10. "Those who reject faith and deny our signs will be companions of Hell-fire."
Notice that it is action verbs "reject" and "deny" that bring on this fate; not a passive "belonging" or "not belonging" to a named "religion." The kaafir is judged by Allah as one who denies and rejects; merely belonging to a Christian or Muslim religion as we define them today is not the criterion for entering hell or paradise. So in the first place, you do not really know and cannot arbitrarily presume that your nominally Christian relatives will necessarily go to hell; they may one day accept Islam.
It is also made clear in the Quran that active believers (believing Muslims who act accordingly) are destined for paradise (al-janna or the Garden).
From Surat Al-Baqara 2:82:
82. "But those who have faith and work righteousness, they are companions of the Garden: Therein shall they abide (For ever)."
Here it is also not said that those who belong to the Islamic faith will automatically go to paradise. Rather again it is their hearts and deeds— "have faith" and "work righteousness" — that is the criterion for their fate. Muslims are presumed to believe in Allah, the prophet and the last day; but for many nominally Muslim people, they actually believe by being born Muslim they will enter paradise based on that lineage. This is not what determines their fate as mentioned in the Quran but rather it is based on what is in their hearts (attitude) as made manifest in their actions.
Both these situations refer to the Hereafter; neither is said to be determined by earthly relationships, but rather solely on one's own faith and one's own actions. Each is responsible for him/herself.
From Surat Al-Muddathir 74:38:
38. "Every soul (self) will be held responsible for its deeds."
Here the word "nafs" or "soul/self" refers to an individual. This is further emphasized in Surat Maryam 19:80:
- "To Us shall return all that he talks of and he shall appear before Us bare and alone." (My emphasis)
This refers to the Day of Resurrection. The phrase "all that he talks of" refers to false statements made by a kaafir mentioned previously. What is clear is that people return to Allah with only their deeds and what they said "fardan" which means entirely alone, with no other support—no relatives.
This makes the "relative" issue even clearer:
Surat Faatir 35:18
And none can carry the load of another; and even if it calls on another to bear part of its load, no other can carry any part of it, even if they were related. You will only be able to warn those who fear their Lord while unseen, and establish regular Salat. And whoever purifies himself does so for the benefit of his own soul; and the destination (of all) is to Allah.
However, if relatives are believers and their actions demonstrate that faith, they will be joined in paradise.
Surat al-Tur 52:21
21. And those who believe and whose families follow them in Faith,- to them shall We join their families: Nor shall We deprive them (of the fruit) of aught of their works: (Yet) is each individual in pledge for his deeds.
As to where your Christian relatives will spend eternity, you cannot really know. First, they may become Muslim before they die. Second, their fate depends first on whether they were in fact disbelievers in their hearts and their actions reflected that, something only Allah will judge on the Day of Judgment. If so adjudicated by the Almighty, they would be unable to escape from hell even though they love a Muslim. If you are, on the other hand, a believing Muslim who has applied your faith in acts of righteousness, then you cannot be sent to hell either even if you love a kaafir. Prophet Mohammad loved his uncle who persisted in disbelief until he died. Love did not affect their akhira or fate in the Hereafter.
There is no danger whatsoever of going to hell in order to be with those you love; however, sometimes such relationships can result in communication between both even though one is irrevocably in paradise and the other in hell. But this will not bring them together; and Allah will remove all feelings of regret or sadness in the believers that in this life we might think could result from such separation.
The hadeeth quoted must be understood in context; it is a response to a specific question, and although the literal words can be applied non-specifically, one cannot make an unequivocal pronouncement out of context. The answer meant simply that the person inquiring will be resurrected to be with those he loves, namely the prophet and also the "face" and acceptance of Allah, not that people condemned to hell will somehow end up with faithful Muslims destined for paradise because they loved them in this life or that either will be resurrected in "the wrong place"—that would contradict the Quran and even suggest wrongly that mistakes could be made by the Almighty, which implication would be a gross sin.
I hope this helps.