I hope I'm not being insensitive here but here's my 2 cents...
This tattoo wouldn't be a tribute to an actual person rather a reminder of a horrible event. This child wasn't even around long enough to remember except the few pieces of memorabilia you are trying to hold on to, like footprints.
What happens if your sister gets pregnant next week and has a complication free pregnancy, resulting in a real living breathing niece? If that child were to die a year later I could justify this type of tribute tattoo, as you would have memories of an actual person with a soul and personality, not just an event that left you hurt and sad.
Although I think you were being insensitive I will give you the fact that it would be more a reminder to the event than it would be a tribute. The more I think about it the more I agree. But to say that the child was not even around long enough to remember is wrong. To say that she was not a real living breathing niece or that she didn't have a soul is completely insensitive. You may have your beliefs but I also have mine and I believe that life begins at conception. You might want to realize that other people do have different beliefs before you go ranting on. Sorry if I'm being insensitive..