What should my daughter call my husband?

Sheila
I got married 4 months ago to my 3 month old son's father. I also have a 5 year old daughter from a previous relationship. Her daddy is very active in her life. She sees him every weekend and most school breaks. My husband has been in my daughter's life since she was two and they are close as well. My issue is that when I got married my daughter started calling my husband daddy. I told explained to her that it might hurt her daddy's feelings and that she can make up a name for my husband instead. She chose "dad" because he is her step-dad. Now my ex and his family are in a big uproar and saying that it's wrong and she should only call my husband by his name. I never wanted my daughter to feel like she didn't belong in our family just because I married her brother's dad. She is a smart and strong little girl but still sensitive. I don't want to make her feel like she has done something wrong. What should I do?