I am going against the grain here- I think YANBU. I think her heart may be in the right place but she is actually just putting another form of pressure to conform onto a young child. I don’t think that that behaviour or attitude should be applauded at all. True feminism is about supporting the right to choose for ourselves regarding lifestyle/education/career, relationships, the way we dress etc- and for our choices to be viewed as valid and respected. So if a woman chooses to do something more traditionally ‘feminine’ that should be every bit as valid, valued and respected as those choosing another path.
Your SIL seems to imply that only the non-stereotypical feminine choices (e.g. football instead of baking, not wearing a nice dress) are acceptable ones. It’s a subtle suggestion that what she is doing/wearing is “not right” and traditionally ‘girly”/feminine things are less than/less important/unworthy of her time or consideration. In reality, many 3 year olds (of both genders) would prefer baking with mum to a football match (a match can be quite long for an young child, especially if it’s cold and football has not caught their interest) and lots of little girls like wearing sparkly dresses. And there is not a thing wrong with either of those. It sounds like your SIL is a bit tedious and forthright about her ideas of what feminism is and how she thinks girls/women should behave or dress.
It’s not like OP’s DD was desperate to go to the football but OP forbade it as unbecoming to a girl and told her she must stay at home to bake so the boys could have something nice to eat when they got in from their football match. Suggesting a little girl wear a pretty dress to a special event is really not damaging- suggesting she can ignore her parent’s instructions if she wishes (not at 3 years old, she can’t- and they don’t usually have the ability to distinguish between import vs less important instructions/situations at that age) is undermining OP and has potential to cause issues for the child. Also, children do need to learn to dress appropriately- going out in dirty clothes to an event is not showing respect to your host(s) and is generally viewed as less than socially acceptable- it’s part of learning how to function around others. One instance of SIL overstepping is neither here nor there in the grand scheme of things, but if kept up and often enough it could be a problem. It’s simply not appropriate for SIL to interfere like that. If it happens again, address it with her directly and ask her to stop.
I also object to the idea that we should not tell our children they are pretty/handsome- obviously, it isn’t the only thing we should tell them or make it into a huge deal, but good lord are we not allowed to say it at all now? I like being told I look nice. I often tell my 15 year old she looks pretty/beautiful/really suits a new hairstyle/colour of dress or top etc. I’m not making her looks the most important aspect about her, I’m paying her a compliment. We have far more discussions about more important things relating to her education, ambitions, hobbies, friendships, current affairs etc- but I still think it’s absolutely fine- possibly even important- to say a child looks attractive (whatever their gender) from time to time, especially if it’s for a special event or they’ve made a specific effort.
I would point out many women do enjoy looking nice- and that can include putting on make up- not for men, but for themselves. Feminism and make up and wanting to look attractive are really not mutually exclusive. I like putting on make-up. I don’t wear a full face of make up day-to-day (though it;s fine if you want to, surely), but I really enjoy it as part of getting ready for going out/for a special occasion. I like looking my best, in clothes that suit me. My dd and her friends enjoy putting on/wearing make-up- they sometimes watch videos about it/practice using it as inspiration, including special effects and “horror” makeup and also what can look like watercolour and actual art using faces/make up- so it can be a form of art, in it’s own way. Even the more day to day “beauty” make-up is something they enjoy. Yet, these same girls are not defined by that or the way they look- people are more than capable of being interested in how they look, whilst not being indoctrinated or oppressed. These girls all have wide interests and have hobbies including playing sports (cross country running, rugby, hockey, eventing, gymnastics and mountain biking); several enjoy outdoor activities like canyoning and rock climbing; some are really into music and singing; one is a talented dancer. They also quite enjoy baking in my kitchen quite frequently- even if they are less keen on tidying up! My daughter spends quite a bit of time covered in mud, in far from “girly clothes”, picking up bruises etc. But still really enjoys getting dressed up in a nice outfit (often a dress) and doing her hair and make-up.
Honestly, I don’t think OP’s actions are reflective of true feminism and I would ask her to take a step back. It sounds like she feels the need to impart her own ideas and correct you every time you say or do something that she doesn’t agree with/has a different take on- it’s really not her place. If she has a real concern she should come out and say it directly, rather than this not so subtle undermining she is currently attempting- perhaps unconsciously.