"Loco filiae" is Latin for "place of the daughter". This concept was used in Roman Law, in attention to one of the ways a person could be under custody of the paterfamilias, or under his power.
The paterfamilias was the main chief of the family. None of the other members had any power over him; he held the right to dictate the behavior of the members of the family under his custody, and his own. These members could be his wife, his children, and other people's children (we can't speak of slaves in this case, since they were not considered people, but things), unless any of them became emancipated. Women usually always belonged to a paterfamilias, since they were generally daughters of someone, or wives of someone.
When a woman got married, she had to decide whether she wanted to stay under her father's custody, or leave her family and submit herself to her husband's custody. If she decided for the latter, her husband may not be the paterfamilias himself; if his father was still alive and he hadn't been emancipated, he was the paterfamilias. So, in this case, the wife would enter her husband's family under the custody of her father-in-law. This place in the family was the one Romans called loco filiae, place of the daughter, since the paterfamilias daughter-in-law became his daughter as far as civil law was concerned, and also her own husband's sister.
Except, and Romans were so good at foreseeing this stuff, they gave her that special name because if she had actually attained the status of a daughter, Roman civil law would judge this an incestuous marriage, and nullify it.