I know how you feel. Well, I did.
I have always wanted children and when people fell pregnant around me I used to get s sad. I hated other people for being pregnant yet I still spent time on websites about pregnancy and stuff.
Your time WILL come. Your baby will be planned, and wanted. Stuff your SIL - SIL's are evil anyway!

You'll be fine. Its understandable hun, don't make yourself feel bad about it! Are you TTC now?
If you are, then know that your day will come, and soon. Best of luck, hun x