VF3 works the same way that VF2 did with respect to remarriage: if the couple had children, then remarriage isn't an option. It's kind of complicated, but it's because of how the game manages the Family Tree.
There are a couple of ways you could proceed:
1. If you're still in the first generation, you could evict Trishina (it sounds worse than it is) and adopt a new peep, who would keep all of the game progress you have made so far - money, rooms, food, etc. Then you basically play the first generation from where you left off, but with the new peep.
2. If you have enough money, you could adopt a child for Trishina (or hope you get one of the "orphan" events) that would provide an heir for moving to the new generation. If you don't have enough money, you could always spend some time picking up the collectibles to help with that.
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Barbara
Unicorn
Last Day of Work