What TreadSoftly and Munchcrunch say.
There are a lot of threads which boil down to this on MN - one adult sibling enabled in financial and often other types of dependency by parents of that adult, and siblings frustrated and possibly resentful - both human emotions, all very saintly to rise above them but it is human to be frustrated and a little bit resentful about your parents heavily indulging one sibling over the others whilst the others take the brunt of the moaning about it, whether you are 15 or 45...
I think there is often a lot more going on behind the scenes, with parents unable to stop because one or both of them convinces themselves their adult child still needs them, and this gives them a slightly martyred kind of pleasure or fulfils an emotional need, so to some degree it is a co-dependency. There is also often a family mythology around that one adult sibling being in some way emotionally fragile for some real but historical or blown out of proportion reason... Always there is the family culture of not "rocking the boat" by talking about things that could upset people even if they clearly need talking about...
If you or your DH talk to your sister you will achieve nothing except possibly sending her running to her parents like a child who's brother has called her farty pants, and might even end up with your in-laws telling your DH to apologise for upsetting her her, leading to another AIBU not to encourage DH to apologise to his sister just to keep the peace...
I think you probably have to keep out of it unless you just want to clear the air once and for all, accepting that the end result will be no change in the financial draining and you and your DH nudged further to the outskirts of the extended family and given a bit of a cold shoulder for breaking unwritten rules about what not to talk about...
If you can be zen about it then hats off to you, but you and your DH would be only human to be frustrated and cross about it...