Do you pay your MIL to watch your son? If not, I wouldn’t go down the path of claiming that the days your brother watches count as your days, because then your wife gets to count all the days her mother watches as her days.
As for FMLA leave, does it matter if she uses it for her father? If she didn’t, wouldn’t MIL have to take that time to care for him, leaving you and your wife needing to watch your son.
Reading your posts, I get the impression you do expect your wife to be responsible for child care and that when you do so, it’s doing her a favor. That might work in marriages with a SAHM, but when both parents work, it’s not an equitable or fair arrangement.
I see your point about the MIL/Brother thing. My MIL agreed to watch him for the 1st two years out of the kindness of her heart in spending time with her grandchild, but because of her recent health complications, and FIL's dementia in the last month, She can no longer watch S1 going forward which I completely understand plus the added pressure of FIL.. But Rose in other words. Since IHS in Jan. and established informal monthly custodial arrangement. When I take off of work to watch S1 whether it be voluntarily due to inclimate weather, or whether it be to W's circumstances with MIL OR FIL or W's own health conditions. It doesn't count with her. ( Between Feb-Mar. W went for numerous bariatric consultations all the way up to pre-o.p. had 2nd thoughts about it for her own personal reasons, then a month later gaslighted me, and blamed the entire reason why she didn't follow through with bariatric weight loss surgery on me, because HER FEELINGS. felt I wouldn't be able to care for S1 for a month of her anticipated recovery.) This is months before MIL and FIL's health issues and surgeries. I had to take a maybe few half days here and there during that time to pick W up from endoscopy from hospital, and other consultations. Not a huge sacrifice on my part, but also at a time when there was a lot of inclimate weather, snow and rain where I was voluntarily watching S1 to give W and MIL a break. So I am losing money to pay financial obligation two fold. In addition to that, I have taken off additional days over past two months to watch S1 while MIL was going through her issues. But again anything I do is not acknowledged, considered, or is equitible to her.
Everyone on here months ago has told me "Well if she fired you as her H" then it is both of your responsibility to figure out your own arrangements so that is what I did. I left her to figure out hers, and mine my own. But that doesn't count torwards anything with her. In other words she "wants equal suffrage" from me for all the days she had to miss work related to child care due to her parents illnesses as of recently. Then she was unclear about terms of me going away for two weeks, staying "we need the money" but then wanted me to make up for "my nights" that I missed while I was away after the fact. Which I am doing.
I know it sounds like I am,being cold and legalistic, but if this isn't cake eating, then I don't know what is? Maybe Im totally misguided by this forum and the suggestions here, and have only made things worse by following them I wonder sometimes? Its hard to interpret someone's ongoing scenario when your not experiencing all the craziness everyday/month/year for yourself. Maybe I took the whole "cake eating" theory principle here too literally? It seems like all the advice I have taken here dating back to Jan starting with by reclaiming the MBR and other actions has deteriorated relations even further, not that it matters now? Who knows?