Should I keep trying or wait?

Sherri • With the help of Glow, now pregnant with my third and final baby! :)
Long story short: I have a 8 year old girl, and a 5 year old boy and are now trying for baby number 3! My problem is, is that my sister is also trying to get pregnant and HAS been trying for 5 years now.... She (with a doctors help) FINALLY got pregnant only to miscarry. :( So now I'm torn. I want another baby but I don't want to hurt her emotionally if it happens for me and not her. ADVICE PLEASE! Thanks in advance!