breastfeeding in front of step children

I need advise, I plan on breast feeding and my two teens are mortified of this but are adjusting. I also have two lovely little girls that he brought from a previous marriage. 5 and 2. How should I approach this with them? I have good communication with their mom, should I talk to her about it? She has loved me till I got pregnant so some things she is great and some times she is spiteful and I just never know who I am going to get. PLEASE HELP! I love these girls and want them comfortable at all times. My focus is the 5 year old.