My (24) younger sister (17) is planning to have a baby with her boyfriend (19) as soon as she turns 18. She will not listen to me or our parents, and her boyfriend’s family is supporting her decision.

Well it is her choice plus she has the support of the other family. Sure it is unwise and in my opinion one should get married and enjoy their partner first before throwing a baby into the mix. I told the same thing to my older brother's(M30) young 18 year old bride. I told her best to relax and get to know each others faults and strengths, but she wanted what she wanted and has always been headstrong about getting what she wants. She got my brother didnt she and she wanted hom since she was 13. You can't tell them what to do especially at this age. The best you can do is guide her and support her. No "I told you so" once the child raising gets tough! She will know when the time comes and the best case scenario she will grow stronger in the process and not lose her marbles or her will to carry on the mothering.

/r/relationships Thread