Nellie17, s always some very good points made here by GNers.
Just a few things occurred to me in addition to what's already been said that may affect your cousin's comment.
Your cousin is clearly put out and and rather than addressing any concerns she might have about you help, motivation to help, she made a cutting comment. Was it in jest? If you thought she was a jesting kind of person you might not have felt so hurt so I don't think it was said in jest.
You said you stayed away before your aunt was widowed because you uncle wasn't very nice.
Is your cousin more like her dad than her mum? Was her comment more along the line of what her dad would have said?
Also, your cousin is single, therefore, maybe the process of caring isn't as natural to her as it is to you?
Could it be your life experience/willingness to research, to help, is something she hadn't thought about doing for her mum?
As others have said I wonder how much of her comment is driven by guilt.
Maybe your cousin would like to have the kind of support you're giving your aunt but doesn't show it. Maybe she would benefit from the advice/information you've given your aunt and didn't know it until you got it for your aunt.
Maybe your aunt hasn't had as much care/support in the past so your aunt is singing your praises and that is clearly annoying your cousin.
So many questions for you to ask yourself that might give you some insight into what motivated your cousin to make her remark. Not that more insight will change what happened but it may help you recover from your hurt and help you in any future contact with your cousin.
As for talking to your cousin about it? Unless there is a natural situation where you feel you must speak up I'd leave it and pay attention to what happens next.
I'd visit when she's not there and ask your aunt if she feels helped by you or if she feels you are interfering.
Once you are clear about how your aunt feels you can make your choice about how you go forward.
Also once you are clear about how your aunt sees your visit/input you might give yourself the best chance of taking care of your and your aunt's feeling in any future situations.
Wishing you all the best and may you recover from your hurt quickly and guard agains any future hurt.