I think Roxy Marie was introduced around the time that Richard Hunt died, but I think I've also seen some say that the family relationship between Biff and Roxy took a year or two before it was established.
Roxy Marie did stay on the show for most of the 1990s. I remember her being in a season 29 episode where the Furry Arms Hotel gets infested with fleas (or was that episode a repeat from an earlier season?).
As I mentioned, there's at least three season 25 appearances by Sully (where he and Biff have scenes in the park, they demonstrate some, more, and all at Finders Keepers, and they visit the day care center). But then Biff also had quite a few scenes without Sully, though he's always had a lot more solo appearances. There's the episode where he got his thumb stuck in a bowling ball, which featured neither Sully or Roxy Marie (the first half of the episode paired him with Telly, and then his wife Celeste appeared with him for the rest of the episode). There were quite a few segments with Biff and Roxy, and in the season 25 premiere, Biff is involved with building the wall over the new park, with Sully no where to be seen (it's made clear that a construction crew is there with Biff, I'd like to think Sully was there but just not seen or mentioned).
And then after season 25, there was an episode where it's Biff's birthday, and Sully doesn't appear at all. That's kind of sad, though it does make a little sense since Biff didn't want a birthday party and Roxy Marie and Elmo put together a party around the last minute (Sully might have been too busy to go to a suddenly put-together party), though it's good that Sully was briefly mentioned in the episode. And there's also an episode where Biff waits for Sully to show up on Sesame Street, and Sully did show up at the end (after a misunderstanding on what street to meet at). And their last big appearance was in a season 30 episode where they drill on the street.
I also find it a little sad how Biff appeared in both Sesame Street policy films that played at Sony/Loews Theaters, but Sully was nowhere to be seen. Especially "Don't Forget to Watch the Movie", which fits in many characters.
If i'm not mistaken, I think I read somewhere that the reason Sully didn't appear much in the 90s was because they were trying to gradually phase Biff out of the show. And I heard that at some point, the writers were even going to replace him with his niece, Roxie Marie.
That doesn't make a lot of sense. So they were phasing out Sully because they were planning to phase out Biff, but kept Biff without Sully? Why not just phase both out at the time? And then replace him with his own niece, who seems to be an honorary replacement for Sully? Weird.