ANSWER:

You must not listen to her in this regard as there’s no obedience to the created at the cost of disobeying the Creator. 

Explain in the most respectful manner to her that you cannot attend and that she should not allow your sister to attend as well. 

Sheikh,there’s grand annual dinner of myuniversity,which is not segregated and have music 2,my sister is going as she wanted but i donot want to go,my mum is forcing me to go so that my sister will not be alone cause the event will over at night so it’ll be difficult for her to travel alone @ night,how should i convience my mum and sister that i don’t want to go

Fiqh of the family | April 9, 2018 8:49 am
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I know that we are not supposed to obey parents when they call us towards sin, but what about if they stop us from performing good deeds or keep us restrained to the point that we are not able to become better and useful Muslims? Especially, because we will be questioned about our potential, but if our parents stop us then what must be done?
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Shiekh, Doesn’t Allah forgive the one who misbehaves with his parents and then seeks forgiveness to Allah? as he feels bad after that.