TTC For nearly two years

Me and my partner have been trying to get pregnant for nearly 2 years now I already have 3 kids from previous relationships my partner wants his own child more than anything he’s having fertility treatment but awaiting results he’s told me that we are enough for him but I’m scared that if he finds out he can’t have children he will leave I’m so scared and trying to support him and then the other day we found out my sister who hasn’t been trying is pregnant again and the look on his face was happy but also disappointed that it wasn’t us please can anyone give me advice