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

Praise be to Allah.

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.