First of all, congrats on the newborn! And!! Kudos to you!! I know it is not easy being a ebm mum as I have a couple of friends who are going through this as well. More so when you have family members acting against your wishes.
I would suggest getting your husband to handle your mother-in-law. Get him to explain to her the benefits of breast milk and how both of you wish to continue with this option now. Assure her that you are supplementing with formula as none of you are trying to let the baby go hungry.
You can take a look at this article (and the links in it for the various parts) to understand the causes of low supply. Identifying the cause may help you tackle the issue more efficiently:
http://livingwithlowmilksupply.com/low-milk-supply-in-breastfeeding
I hope you find these information useful. Can only offer you my moral support! Hang in there!!
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