TTC stepmom, a somewhat rant

Ashlea • 27 years old... Married the love of my life 5.20.15 and gained 3 amazing step kids, baby #1 due May 2017
So I am a step mother to 3 wonderful children, ages 3, 8, and 10. My husband and I have been trying to conceive for almost 5 months now with no luck and each month AF comes I am completely devastated but a part of me wonders if this is some sort of sign that I'm only supposed to be a step mother and not have kids of my own. Then it brings on the question of are we being fair to the kids trying to bring another child into the mix when we are going through a custody battle and all the stress added from the bad relationship between my husband and his exgirlfriend (the kids mother). The kids already feel unwanted by her and like they don't exist in her house, is it fair of me to bring a baby, that is going to take some attention away from them, into their lives?