Dealing with baby envy ?

My sister  accidently got pregnant and didn't know till she was 20 weeks and had the baby this month. However I feel really jealous as me and my s/o have been trying for over a year now. She wasn't ready for a baby and even said it herself and I love my niece of course but can't help but to be extremely jealous especially because my mom keeps telling me that she's glad that I'm not ruining my life with a child.  ( we're keeping ttc on the down low)  how can I deal with these feelings?