It's very sad isn't it? That the loss of someone who held the family together doesn't strengthen the bond, but often severs it. Money and things have ended what may have been perceived as the closest of relationships, many times.
You must consider that your nieces may be sending you gifts because it makes them feel good. If this is the case, taking away that opportunity will only hurt them and possibly cause resentment.
Upon receiving your next gift, send them a formal 'thank you' card with a note expressing that you will never be able to tell them just how much you love their thoughtfulness. Then let them know that in lieu of gifts to you, making a donation to a charity they are passionate about will be gift enough for you. If they'd like to send an old fashioned card through the old fashioned mail you would enjoy that.
You might compose a note similar to this:
Jane, I have always loved how thoughtful your gifts are to me. At this point in my life I really don't need much and what fills my heart is caring for those in need. Whether it be animal care or any number of human helpers, please donate what you would spend on a gift to me to any charity near and dear to you. That will be my 'gift'. If you'd like to send me a card I'd enjoy that. Knowing you took the time to pick one out just for me is a gift of time that will always be priceless"
Then, if gifts continue - just say thank you and enjoy it. You've done all you can do