Congratulate your sister and be happy or happy for her, I get that it’s hard but it’s not really that bad, my mum had her first child at 19 but she was pregnant at 18, my mums now 41 and my sister is 22, it doesnt really matter how old she is, as long as it’s legal, my mums life wasn’t ruined, my friends mum gave birth at 15 and went to uni, it’s okay