Wa `alaykum as-Salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Hence, if someone is eating or drinking shortly before the adhan of Fajr, he/she must stop on hearing the adhan.
As for your case, you have done a big mistake by drinking and your mother-in-law has done a bigger one by eating and drinking after the adhan of Fajr.
But, as long as you have done this by mistake, i.e. not intentionally or on purpose, there is no problem for you both. You just have to ask Allah Almighty for forgiveness (istighfar) and you must make up for that day after Ramadan.
It was narrated that the Prophet (peace be upon him) said, “My Ummah will not be held accountable for what have done by mistake, what they forgot, and what they had been forced upon.” (At-Tabarani)
Mind, also, to make up for any day on which you have done similar mistakes.
Allah Almighty was best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.