Twins and triplets arrive at random. Last time I played I think it wasn't until I bought all the health or fertility tech levels til I even got twins. Other times I played I had a few twins earlier, but no triplets til almost finished the game. They did come in handy then when I still had some collectibles to find, need babies then.
Maybe some of the tea potions help, didn't try that, but I'm not one to want many young'uns same time since they eat too then I have to train them all at jobs. I enjoy all the VV games more keeping populationn down to 20 or less. Especially when they run out of tombstones (50 I think) and on VV2 there's only 50 totems so when you get more elders they're out of luck.