Link your partner's family
In Studio → Family → Link a family, send the link to someone who runs the other family's albums. Once they accept, your trees show together. Albums stay private: you share chapters with their family one by one. You can unlink any time.
How far your tree reaches
Only families we link with directly keeps your people off the trees of families linked to those (your sister-in-law's in-laws, say).
Same family?
Started two trees that are really one family? In Studio → Family → Same family?, join them: one tree, and anyone in both becomes one card. Albums stay as private as they were. This can't be undone.