Sandy,
On our wedding day, my hubby reached my house late, so we ended up leaving my house at around 9am. My MIL had wanted us to reach her house before 9am, so when we reached at 9 plus, she grumbled a little at first but by the end of the day, she was beaming with happiness and busy entertaining her relatives. So try to give in to your MIL, but my guess is even if you are late, she'll soon forget about it. If you are paiseh to your sisters, perhaps you can remind them to sleep early the nite before?