Fifteen years ago, shortly after I met my dh, he told me about his family background and about his ongoing financial commitments to his mum and sister, who would otherwise be living in poverty (overseas). I entered into a committed relationship with dh in full knowledge of these commitments, and I can honestly say that I have never once questioned or resented them.
At present, he probably spends around £10,000 a year on supporting extended family - there have been years when it has been more. We are in the fortunate position of being able to afford this, but it does impact on our lifestyle, but that's fine - I can manage without a few of my wants in order to ensure that my sil and her kids have access to what they need.
dh's love and support for his family is one of the things I most admire about him, and I struggle to see how anyone could see this as a negative unless out of pure selfishness and greed. What on earth is dysfunctional about helping a family member who needs it?!
During the years that we have been together, there have been a few periods in which dh hasn't been earning, and I have been entirely happy to continue supporting his family from my income. Yes, it might mean a few less toys and luxuries for my dd, but at least her cousins have a roof over their heads and the chance to go to school.
op, I think you need to accept that your dp wishes to support his sister and this will probably never change. It doesn't mean that your dcs will go hungry, but he may well think that supporting his sister is a higher priority than luxuries for his own family, and that's a perfectly reasonable point of view - you just need to decide if you can live with that, and if you can't, then you need to find someone else to fund the lifestyle that you're after.