Hello! It is massively confusing, but here is what I think is the position.
George's father is Will - yes. He is not George's uncle by marriage.
Emma is George's mother - yes. She is not George's aunt by marriage.
Emma and Will were married but have been divorced for a long time. Ed, Emma's second husband, is Will's younger brother. There was at one point suspicion that he was George's biological father, but Will got a DNA test done and proved that he was the father. Ed is therefore both George's stepfather and his uncle.
Keira is the daughter of Ed and Emma, so half-sister to George and niece to Will.
Poppy is the daughter of Will and Nic (see post above), so also half-sister to George, and niece to Ed and Em.
Poppy and Keira are cousins, yes. Although they are both George's half-sisters, I don't think they can be described as stepsisters, but to be honest my head is spinning now and I'm not sure.
Horrobins: as Totters has outlined, we only know about the Horrobins who are direct descendants of Ivy and Bert. Their children are Susan, Clive, Keith, Gary, Stuart, Tracy (I think). Not sure Gary and Stuart have ever been heard on air. Gary used to live with Bert and Tracy but moved away when he got a job elsewhere. Keith featured briefly at one point. Clive is an out and out wrong'un and has reappeared periodically to wreak havoc. He has spent a large part of his adult life in prison. Keith and Stuart also have criminal records. Susan served a short prison sentence long ago for hiding Clive when he was on the run after holding up the Post Office at gunpoint (before she was postmistress), which she only did because he threatened to hurt Emma and Christopher.
Next generation of Horrobins:
Susan married Neil and they have two children, Emma and Chris. Emma is the one connecting the Horrobins and the Grundys (see above). Emma has two children, George and Keira. Chris used to be married to Alice and they have a daughter, Martha. This makes George and Martha first cousins. Alice used to be George's aunt by marriage.
Clive had a girlfriend long ago called Sharon Richards, and they had a daughter, Kylie. Clive was not interested in family life and has had nothing to do with either Sharon or Kylie for decades. Sharon moved to Leeds with Kylie and got married. Kylie became a psychologist, I think. Sharon also had a son, Johnnie, who was eventually told that his father was not Sharon's husband but John Archer, eldest son of Pat and Tony, who died in a tractor accident something like 30 years ago. He never knew Sharon was pregnant and neither did anyone else in Ambridge until Johnnie turned up. He was a very good character but seems to have moved on now. Implausibly he is supposed to be working as a crew member on superyachts owned by the very rich. Many of us are concerned that the scriptwriters might have forgotten about him as he should really be in line to inherit a substantial chunk of Bridge Farm.
Tracy had a partner (I don't think they were married) whose name I forget and they had two children, Chelsea and Brad. They split up a long time ago and Tracy moved back in with her Dad, later moving her partner (now husband) Jazzer in too. Chelsea is a hairdresser and beautician who for some reason also spends a lot of time working as a waitress. Brad is an undergraduate at the University of Felpersham studying Maths, working part-time at Grey Gables. Brad and Chelsea are first cousins to Emma and Chris, so first cousins once removed (?) to George.
I don't think we know anything about any other Horrobins in the next generation.