I think I am but my husband disagrees!
So his reasoning is that my sister and her partner never (hardly ever - there have been a few times) given our children (DD1 DS1 and DS2) presents despite asking me every year what to get nothing is ever given. My husband feels disappointed as every year on FB there are pictures of their children surrounded by piles of presents - more than we would buy for our own - yet can't manage to give ours a £1 colouring book for example. She doesn't ever remember the children's birthdays either not even to send them a card - I don't post them on FB so I don't think she even knows when they are.
So there is a massive backstory and I'll try to be brief. My sister is a low income family, her partner is on long term sick because of depression (we are talking nearly 10 years) and my sister is in a low income job so they live in a quite grotty and small rented flat. Over the years my sisters partners (MSP from now on) children from previous relationships (currently DD1 age 25 with baby, DS1 23 and DD2 19) have also come to live in this 2 bed place which is obviously really cramped. They used to both smoke heavily indoors and children's clothes reeked. When my sister did work full time her DD2 suffered form Vit D deficiency and problems walking as she was never taken out (also developed asthma). There are many things here that upset me and over the years I have advised and things have improved in their home - particularly the smoking! But this may be another thread. MSP also smokes weed often despite my sister asking my dad weekly for cash for food (they have also used the food bank many times).
So moving on, over the few years I have encouraged my sister to come away on holiday with her children (MSP never comes and neither does my DH) to help get them into a different environment as well as spending time with my nieces. She will pay half the holiday rental and half food as we go self catering - so really she is paying just to feed hart family. I pay for fuel and extras such as fish and chips, ice cream, outings alcohol. I do this because I care for my family and want them to experience and enjoy some of the things they cannot have and get them out to see new places and do new things. Because their home life isn't ideal I really want to give them some childhood experiences they would have otherwise not have had. Problem is last few holidays I've ended up paying for the food in addition to the other costs and my sister is a loud and constant shouter and even my DD1 has found it tough when they join us for the week. So I've reached the point where I don't want to go away with them any more and my husband is adamant my sister is taking advantage of me one example is that she's has not payed me back for food from out last trip this summer.
FWIW DH and I are not hugely wealthy. Although I am a graduate and have a professional job now we also live in a small house but our hard work has meant this is well set up for our family and we can afford a few trips/holidays a year. Because we are financially okish I feel that it is important to help my sister and her children who are in a much more difficult position but my husband says I should stop because I am being taken advantage of.
My husband wanted to add a few other things - we haven't been able to develop a relationship with MSP as he always goes out or stays in his room when we visit. There is a feeling in the family that his depression is an excuse to not work and claim benefits. He has an expensive mountain bike (bought by his parents also low income family) and is often is out with mates on that leaving my sister to run the home and look after the children as well as work. He has recently just bought a car despite having no need for this - my sister doesn't drive, her work and school are within 10 mins walk from home and good public transport to see friends/family (MSP refused to use this). Somehow they plan to afford to get married next year too. 
So the AIBU is should I do as my husband says and not get the children presents for Christmas/birthdays or should I do what I feel is right - not give gifts but take them on outings and do activities instead as I think not giving anything would ultimately just punish them.
And a medal for getting to the end