“Please Help, Our Landlord’s Son Has Gotten My Sister Pregnant, Again”

This is the complaints of a confused NGLatest reader. Please read and and advise duly:

"Please Help, Our Landlord's Son Has Gotten My Sister Pregnant, Again"

It all started when my dad died and we moved to a new apartment. My immediate elder sister who was 21yrs old by then started hanging out with our new landlord’s son, after few months of our stay there.
There was a day I caught them in a compromising position right inside our house and I summoned courage n told him not to try dat in our house again, and he left. Few months after that, my sister got pregnant and they decided to keep the baby. She later left with the guy to PH where he was working. Barely 2yrs after that, she got pregnant again and gave birth to another child. By this time, the guy had the money to perform the marriage rite but because of the misunderstanding between them, he told her he’ll only do that when he’s convinced that she’s submissive.
Few yrs later, the guy lost his job and they decided to come back home.
Now during Xmas when my sister visited, I discovered that she had a protruding stomach again which means a third child is coming out of wedlock.
Right now I’m planning to write a letter to him through my sister, informing him that he has between now and December 2016 to perform the marriage rite or he’ll be arrested, but I dnt know if there’s any law that governs that. Please i seriously need advice.

[N/B: I’m the last son in my family. I have 2 elder brothers but they’re not doing anything about this issue]