Happy for her, Sad for me

So my sister told me that she's pregnant with her 4th baby after ttc for 1 month, meanwhile, my husband and I are over here on our bazillionth round of Clomid and still nothing. Yes, I have my daughter who I love and adore more than life itself but I feel like a part of me is missing and I can't do anything more to fill the void. And then my sister gets pregnant with her 4th baby like it's nothing!!! It was so easy for them... I am so excited for her but I can't help but be almost devastated for me.