My sister and I are in almost identical situations, both working part time (3 days), husbands work full time, both have mortgages, I imagine our household incomes are similar but my husband and I probably manage out money a bit better, but basically we (my sister and I) both need to work. The only difference is she has 2 kids and I have 1.
My parents have her son and my daughter on Tuesdays and Thursdays, my niece goes to daycare on those days, then on Wednesdays we both have other care arranged that doesn't involve my parents (though my parents often pick her kids up from daycare on a Wednesday at 4 coz they don't want to make them wait until 5:05pm til my sister can pick them up). My sister sometimes gets asked to work on Fridays so my parents have both of her kids then, and my niece sees an OT a couple of times a month so my parents have my nephew then as well. Today, I asked Mum if she'd be able to have my daughter next Friday as work have asked me to come in for a meeting and to stay for the day, and she told me she doesn't have anything on that day but she'll think about it. Then when my niece and nephew were leaving today, mum said "see you tomorrow when mummy goes to work!" So she's got them tomorrow too.
I go out of my way to clean up after my daughter whenever she's there, always packing away her toys, cleaning the high chair, while when my sister leaves there are her kids toys and clothes and shoes and food EVERYWHERE (if I'm cleaning up my daughters toys in one room, I clean up after her kids too). Mum and Dad say they don't care cleaning up after the kids but I think it's a common courtesy to clean up a mess that your kids have made, especially when they're babysitting for free. I thank my parents profusely every time I pick my daughter up, yet I've never heard my sister thank them and we're usually there at the same time.
I'm absolutely grateful to my parents for taking such good care of my daughter for free, and I also don't expect them to always say yes when I ask them to have her for an extra day, but they've had my niece and nephew at least 3-4 extra days over the last couple of months, and I ask them to do one extra day for me and they say they'll think about it. I really don't think I've given mum any reason to be mad at me and she doesn't seem angry and was chatting to me normally, so I'm not sure why she's so OK with having my sisters kids so much more than my daughter, I guess I am feeling a little bit jealous and also confused :-(
Just had to get that out, not sure if I'm really asking a question or if I just needed to get it off my chest... Thanks for reading :-)