Stina Svensdotter has twins, Jonas and Sara in 1805. Sara dies in 1815. Mom marries in 1807. How do I put Jonas and Sara in FS. Do I put them under Stina and her husband or do I build a separate line? This research is Swedish.
It depends on who the father is. I'm guessing it's not Stina's husband, because then the answer would be obvious -- you would put Jonas and Sara as children of Stina and her husband.
If the father is unknown, then you would put them as children of Stina and an unknown father. Stina would have a couple relationship with her husband (adding a Marriage event to that couple relationship for 1807).
Optionally, you could also add Jonas and Sara as children of Stina and her husband, but if you do this, make sure that you set the relationship type between Sara and the husband as Step (or Adopted, if that was the case) and do the same for Jonas.