TTC #1 and sister is TTC

This is my first month TTC been married a year and we are ready to start a family. My sister has been TTC 18months+ with everything medical okay on there part. It's a very sensitive subject of course. But my mom told me to wait, it's not fair. I told her its our decision and it's sucks for my sister I can't even think if that was me but WE are ready.    And she said well I don't need to know when or if your pregnant. But she sits her and tracks my sister like clock work. I don't know what to do, feeling bummed I can't talk about it... Anyone else been in kinda the same boat.