How to deal with mil and sil while TTC

My husband and I just got married a little over a month ago. His mother is constantly telling me she wants me and him to have a baby. Sometimes she comes to me and puts her hand on my belly (awkward for me) and says "yep you're pregnant". I've taken many storebought tests and had a blood test ran and they were all negative. Its frustrating because my sister in law just got pregnant for the third time and we keep trying and can't seem to get it right. It is heartbreaking to me because I want children and my sil pops them out like it is nothing. Any advice?