This is a tricky situation. Did you communicate directly to your MIL about your intention to latch after work? Try to convince her by telling her your reasons behind (e.g. want to bond with baby etc.). I believe your MIL has no bad intention also. Both your MIL and yourself want the best for the baby.
There was once my grandmother who is taking care of my baby insisted to feed formula milk at night and claimed that it will make baby to feel fuller and sleep well. I convinced her by telling her my intention in breastfeeding (i.e. want to establish supply, giving my best milk to baby and etc.) and finally she understood my intentions.
In addition, you can try to get your MIL to note down the feeding times while you are at work, so both of you can work out a feeding schedule to cater for your need to latch after back from work.
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