Is she sinning by attending wedding parties that involve reprehensible things if she sits far away and helps them with cooking etc?

Firstly: 

It is not permissible to attend wedding parties that include
reprehensible things such as singing accompanied by drums or other musical
instruments, apart from the daff, or mixing between men and women, or other
evils, except for one who is able to denounce that and thinks it most likely
that he will be able to put a stop to the evil by denouncing it. 

Ibn Qudaamah (may Allaah have mercy on him) said: If a person
is invited to a waleemah in which there is sin, such as alcohol and musical
instruments, etc, and he can denounce it and put an end to the evil, then he
must attend and denounce it, because then he will be fulfilling two
obligations: accepting the invitation of his Muslim brother and putting a
stop to evil. But if he is unable to denounce it, then he should not attend.
If he does not know of the evil until he gets there, then he should put a
stop to it, and if he is not able to, then he should leave. Something
similar to this was said by al-Shaafa’i. End quote from al-Mughni,
7/214. 

It says in Fataawa al-Lajnah al-Daa’imah: If the
wedding party is free of evils such as mixing between men and women,
promiscuous singing and so on, or if she attends she will be able to change
the evils, then it is permissible for her to attend in order to share in the
joy. Rather attendance is obligatory if there is some evil that she is able
to put a stop to. 

But if the party involves evils that she cannot denounce,
then it is haraam for her to attend, because of the general meaning of the
verses in which Allaah says (interpretation of the meaning): 

“And leave alone those who take their religion as play and
amusement, and whom the life of this world has deceived. But remind (them)
with it (the Qur’aan) lest a person be given up to destruction for that
which he has earned, when he will find for himself no protector or
intercessor besides Allaah”

[al-An’aam 6:70]  

“And of mankind is he who purchases idle talks (i.e.
music, singing) to mislead (men) from the path of Allaah without knowledge,
and takes it (the path of Allaah, or the Verses of the Qur’aan) by way of
mockery. For such there will be a humiliating torment (in the Hell‑fire)”

[Luqmaan 31:6]

And there are very many
ahaadeeth which condemn singing and musical instruments. End quote from
Fataawa al-Mar’ah, compiled by Muhammad al-Musnad, p. 92 

Secondly: 

If your going to the
wedding and taking part in the cooking etc will not result in your listening
to the evils, or approving of them or helping in them, such as if the place
where the evil actions are being committed is far away from you, or you will
be able to leave before they start the reprehensible actions, then there is
nothing wrong with your going in that case, but you should offer them advice
and explain to them the rulings on these evils, and tell them that it is
haraam to take part in them. 

In his commentary on the
verse (interpretation of the meaning):  

“And it has already
been revealed to you in the Book (this Qur’aan) that when you hear the
Verses of Allaah being denied and mocked at, then sit not with them, until
they engage in a talk other than that; (but if you stayed with them)
certainly in that case you would be like them. Surely, Allaah will collect
the hypocrites and disbelievers all together in Hell”

[al-Nisa’ 4:140]

al-Qurtubi (may Allaah
have mercy on him) said: “then sit not with them, until they engage in a
talk other than that” means, talk other than kufr; “certainly in that
case you would be like them”. This indicates that it is obligatory to
avoid those who commit sin if they openly commit an evil action, because
whoever does not avoid them is approving of what they are doing, and
approving of kufr is itself kufr. Allaah says “certainly in that case you
would be like them”. So whoever sits in a gathering where sin is
committed and does not denounce it has a burden of sin just like them. 

They should be denounced
if they speak of or do sinful things; if a person is not able to denounce
them, then he should get up and leave them, so that he will not be one of
those mentioned in this verse. End quote. 

Al-Sa’di (may Allaah have
mercy on him) said: 

This also includes
attending gatherings where sin and immorality take place, where the commands
and prohibitions of Allaah are taken lightly and the limits that He has set
for His slaves are transgressed. It is forbidden to sit with them “until
they engage in a talk other than that”, i.e., other than disbelieving in
the verses of Allaah and mocking them. “certainly in that case”
means, if you sit with them in those circumstances, “you would be like
them” because you have approved of their kufr and mockery of Islam, and
the one who approves of a sin is like the one who does it. 

To conclude, the one who
attends a gathering in which Allaah is disobeyed is obliged to denounce them
if he is able to, or to leave if he is not. End quote. 

Tafseer al-Sa’di,
p. 217 

And Allaah knows best.