Worried for my niece?

My sister is divorced from her husband and he has a new partner. They have recently announced that she is pregnant and my 9 yr old niece (from his marriage to my sister) is very excited at the thought of a half brother or sister. Literally all she talks about is the pregnancy and the new baby and how she always wanted a brother etc. This is lovely but I am worried about what will happen once this baby is born. My sister's ex has never been a very attentive father and has been known to cancel plans with my niece with only a couple of hours notice, leaving her devastated. He is now saying that he cant take her on holiday in May as planned because his partner is "too tired from the pregnancy to cope with a 9 yr old". They will still be going on holiday together. I have a very bad feeling that once this baby is born he will withdraw from my niece and concentrate on his new family (especially if it is a boy - in the past he has openly expressed his disappointment that my niece is a girl). If this happens she will be totally heartbroken as she adores her dad. How can my sister and I protect my niece from being hurt or rejected? Any advice would be good.