I don't think that anything was ever specifically mentioned on "the Partridge Family" about the exact ages of all the Partridge kids, or about how spaced apart in age they were from each other.
Keith and Lori seemed to have NO more than a year between them. It was very much as if they were twins. Same with Chris and Tracy. Danny was the one who was apparently five years younger than his oldest sibling (Lori) and about 3 years older than his youngest sibling (Chris). Keith was supposed to be under 18 but I guess older than 15 when the series began.

But Tracy seemed to essentially be half Lori's age. It seems when Tracy was about 8 Lori was 16.
There was a significant age difference in the characters but it is still strange that Lori never really talked to Tracy in any episode.
I think that the only scene I recall in which Lori said anything to Tracy was in "Danny And The Mob" when the family were all pool side and Tracy comes up to all of them asking if breakfast was ordered yet and Keith says to her that it would have been ordered by now but everyone was waiting for her to come, and Tracy says she is sorry for being late, but she did not know it would take a long to eat a box of candy, and then Lori says to Tracy "Did you eat all my candy?" And Tracy replies, "just a little."
Of course Tracy almost never talked to Lori either. This is the one episode in which the rare occurrence was shown.

Any one know of any episodes in which Lori even said a few words directed at Tracy, and vice versa?

Lori and Tracey shared a room so I recall some bit interaction between them. During the first year Chris and Danny shared a room but that seems to have been disregarded in later seasons.

I think there was at least one ep where Lori was helping Tracy with her hair, probably several eps. While not an example of talking, it was an example of a sisterly bond.

I think there was at least one ep where Lori was helping Tracy with her hair, probably several eps. While not an example of talking, it was an example of a sisterly bond.

I have not seen each and every PF episode beginning to end, but sure, I guess there was a scene or two here and there of Lori with Tracy, but it was in a "background" type of way and only to give a general sense of typical family life. It was just like when Mrs. Partridge was shown cooking for her family. It was not the focus, but when Mrs. Partridge was not shown cooking there was supposed to be the premise that she normally did cook for her children. Well, of course Mrs. Partridge was shown cooking far more often than Lori was shown interacting with Tracy.

Keith was sometimes shown in school, and at least once was shown as being a member of the basketball team., but little was otherwise said about him interacting with his teammates.

Rubin sometimes mentioned his other clients who he managed, but I think that the only time other clients of his were shown was in "Two For The Show" in which Andy and David Williams were depicted as another act he managed, or that is what I best understood.

Laurie directly speaks to (https://transcripts.foreverdreaming.org/viewtopic.php?t=122031) Tracy in at least, the "Why Did the Music Stop?" (https://www.davidcassidy.com/fansite/TvFilmPages/TPF_S1_E17.html) episode from Season 1:

Laurie: How about you, Tracy?

Tracy: I don't understand the question.