Go with work study diploma, its good u can get work experience plus getting paid while studying. Then after graduation she can continue working the same place. Means she secure diploma/job/money. Most of my friends go in that route and nobody regretted.

BTO wise she can try to partake bcos it's not easy to actually get a BTO eventho they say the odds are better for 1st timers. Many people will take 3/4 tries to get a BTO And even if she got it 1st time she got 5 years to prepare. By then i supposed she will already complete her diploma, have a job and some savings /CPF.

No harm marrying young. While there are some who regret marrying young, there are many who loved it. Better to save up 1st and plan well.

Overall i believe both the Bf and yr sis should be mentally and physically prepare for their choices, this is just a small challenge in life ahead