"Now that she's got this idea that she's going to breastfeed 'her grandbaby', she's started saying that she doesn't want a stronger bond with one baby more than the other (my youngest baby is two months old).
"And since he's already here, she wants to start 'practising' breastfeeding with my baby. I can't even."
To make matters worse, the mum said her mother-in-law asked her to "teach her how to breastfeed as well".
"In hopes it goes well and then she can breastfeed both my baby and [my] sister-in-law's baby, she says she wants to breastfeed them at the same time ... like her eyes gloss over as if it's some dream of her to tandem-feed them.
"This woman is a special kind of delusional. Can women that age really even re-lactate?"
Thankfully the husband shut down the request and told his own mother it was an "inappropriate" request to ask.
Now there is more tension in the family, with the wife believing it'll create a division between her and her mother-in-law.
"Of course this all falls onto me in her eyes, she's mad, saying I'm standing in the way of her having a closer bond with her grandbabies," she wrote.
"I'm the reason. Even though my [husband] was equally grossed out.
"I told her to go talk to her doctor and find a lactation consultant if it's that important to her, but that she will never ever breastfeed any of my babies, no matter what."
After taking to social media, the mother was swamped with supportive comments who backed her stance.
Many said the mother-in-law's breastfeeding request was wrong.
"[This woman] is completely justified in being weirded out by the request," one said.
"It's inappropriate and massively crosses the line. But that should be the case, no matter [the] mother-in-law's age," another wrote.
"Having someone, anyone demand to breastfeed your child and get upset when you say no would be absolutely mortifying, regardless of whether it was someone on in their 90s or a fresh-faced 20-year-old."