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

Question

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.