She doesn't seem to carry any stories herself, judging from the season 8 episodes running on MeTV now. And Patty Petersen was not a good actress, when compared to brother Paul and Shelley Faberes. She had no chemistry with the rest of the cast and just seemed to be a misfit.

All true. They also treat her as though she is their child. I don't remember any episode where they adopted her.

She showed up in the 1962-63 season, she must have been adopted before they skipped to 1966. I wanted to like her, but she was written as mostly annoying. She did have a few good moments. But the episode where Donna and Alex were too slow to tell their friends and co workers they were not going on a vacation, because they didn't want to hurt anyone's feelings wasn't one of them. Trisha had a meltdown when Donna told her they weren't really going, and she ran off crying. Then, when Donna and Alex decided they would really go someplace, discussing it downstairs at a normal speaking level. Trish was upstairs yelling "See Jeff! I told you we were going!" The annoying practice of deleting episodes due to licensing or estate problems didn't help the show any either.