Family dysfunction; spread to niece & nephew?

I've had some disagreements with my sister, over the past year and a half. She's horrendous with money. I've twice denied her insistence for financial support (taking her kids to get footwear; loaning her $800). She already has not repaid our other brother over $1,800 in expenses.

I texted my nephew about what he wanted for his birthday a month beforehand. He never responded to my text, nor acknowledged it. I gave him a gift and a card on his birthday. Granted he had friends with him on his birthday, but he did not acknowledge it over the coming days, nor has he at all.

I texted my niece a few weeks ago, about gifts for her birthday. Again - she never acknowledged the text, nor returned it. I was talking with my brother on Easter, and he said that my niece told him that she would send him some gift suggestions later this week.

At this point, I will likely buy her gift certificates, which is what I did for my nephew. But, at the end of the get-together with my family, I am considering telling my sister I will no longer get gifts for her children.

They are rude and don't return texts They don't acknowledge gifts received. It appears they are adopting their mother's attitude / anger with me, for not financially supporting her (and them). Am I wrong in considering not buying gifts for my niece and nephew any longer? I feel like I'm an outcast, that is being taken for granted - I'm only in their lives when I provide gifts that aren't acknowledged.