He has a sister who is not Muslim; she wants to take a riba-based loan then give him a loan without interest
Question

                My sister, who is not Muslim, is going to borrow 500,000 for me from the bank with interest, and she will pay it off herself with interest. Is it permissible for me to borrow 500,000 from her, and pay back 500,000 to her, without any interest? Please note that she is going to commit herself to paying off the loan with the interest, as a favour to me.
Praise be to Allah.
	Dealing with 
	riba (usury) is emphatically prohibited and is a major sin; the one who does 
	that is subject to a warning of being divinely cursed, which means being 
	cast out from the mercy of Allah. In that regard there is no differentiation 
	between a believer and a disbeliever, because the minor issues of sharia are 
	also addressed to the disbelievers, according to the more correct scholarly 
	view. So what is prohibited for Muslims is also prohibited for them, and 
	they will be punished for it if they do it, in addition to the punishment 
	for disbelief (kufr) and associating others with Allah (shirk).

	An-Nawawi (may 
	Allah have mercy on him) said: The correct view, which is the view of the 
	well-versed scholars and the majority of scholars, is that the minor issues 
	of sharia are also addressed to the disbelievers, so silk is prohibited for 
	them as it is prohibited for Muslims.

	End quote from
	Sharh Muslim (14/39).

	The evidence 
	that the minor issues of sharia are also addressed to the disbelievers is 
	the verses in which Allah, may He be exalted, says (interpretation of the 
	meaning):

	“ ‘What 
	has caused you to enter Hell?’

	They will say: ‘We were not 
	of those who used to offer their Salat (prayers)

	‘Nor we used to feed 
	Al-Miskin (the poor)’”

	[al-Muddaththir 
	74:42-44]

	“And 
	those who do not invoke with Allah another deity or kill the soul which 
	Allah has forbidden [to be killed], except by right, and do not commit 
	unlawful sexual intercourse. And whoever should do that will meet a penalty.

	Multiplied for him is the 
	punishment on the Day of Resurrection, and he will abide therein humiliated”

	[al-Furqaan 
	25:68-69].

	This verse 
	states that the punishment will be multiplied for the one who combines 
	disbelief with murder and zina (unlawful sexual intercourse), unlike the one 
	who combines disbelief with eating and drinking.

	Al-Mustasfa 
	by al-Ghazaali, p. 74

	Similarly, 
	Allah, may He be exalted, condemned the people of Shu‘ayb for their 
	disbelief and giving short measure, and He condemned the people of Loot for 
	their disbelief and homosexuality… This is the view of the Shaafa‘is and 
	Hanbalis according to the correct view, and it is what is implied by the 
	words of Maalik and most of his companions. It is also the view of the Iraqi 
	Hanafi shaykhs.

	Al-Mawsoo‘ah 
	al-Fiqhiyyah 
	(35/20).

	All of this is 
	in addition to the fact that this is also prohibited to them according to 
	their original religious teachings, and it was one of the causes of their 
	becoming subject to Allah’s curse and wrath, as Allah, may He be exalted, 
	says (interpretation of the meaning):

	“For 
	the wrongdoing of the Jews, We made unlawful to them certain good foods 
	which has been lawful to them, and for their hindering many from Allah’s 
	Way;

	And their taking of Riba 
	(usury) though they were forbidden from taking it and their devouring of 
	men’s substance wrongfully (bribery, etc.). And We have prepared for the 
	disbelievers among them a painful torment”

	[an-Nisa’ 
	4:160-161].
	

	Based on that, 
	it is not permissible for you to tell your sister, or help her, or ask her, 
	or approve of her taking out a riba-based loan, and you will be sinning if 
	you do that, because in that case you will be helping her to commit 
	something that is prohibited for her. Allah, may He be exalted, says 
	(interpretation of the meaning):

	“Help 
	you one another in Al-Birr and At-Taqwa (virtue, righteousness and piety); 
	but do not help one another in sin and transgression. And fear Allah. 
	Verily, Allah is Severe in punishment”

	[al-Maa’idah 
	5:2].

	But if she took 
	out the loan without you telling her to do that, or against your refusal and 
	objection, or before you came to know of the prohibition on your telling her 
	to take out the loan, then it is permissible for you to take a loan from her 
	– without interest – because by means of the riba-based loan the money came 
	into her possession, and it is permissible for her to dispose of it, even 
	though she committed a sin by dealing with riba.

	And Allah knows 
	best.