Wow I would be so upset. Good to hear you are starting to feel better about it now. I agree with much of what has already been said and also wanted to say that as violated as you felt/feel the relationship you have with your SIL and your LO's relationship with her aunt are precious. SIL clearly cares for your daughter and having positive strong family bonds (and extended family bonds) in your LOs life is really something that is so precious, irreplaceable, and worth the effort to move on from past misjudged mistakes.
I was unable to bf so my LO was ff. I found people were more than happy to feed my LO when he didn't need (or want) a bottle. The personal, intimate aspect of skin to skin is obviously different when it's a bottle, and in that sense I would find someone bfing my baby more upsetting than someone giving him a bottle at the wrong time. My mum used to feed DS at the wrong time and then re-offer a bottle 2hrs old, full of bacteria, to my baby despite clear instruction of when to feed and when to discard the bottle, it drove me insane and almost ruptured my relationship with my mum for putting my baby's health at risk. If nothing else, at least your SILs milk was fresh, the good stuff.
many hugs, I do hope all the family relationships can be healed with time. x