??????? Letizia's parents have never vacationed in Mallorca with the SRF, neither were the parents of the Infantas (more than 10 years since Elena was married in 95). On the weddings or baptisms, the families of the in-laws (Letizia's or infantas') have never stayed at the official residence. Just as you said, if Cristina didn't want to deal with Letizia's family, why did she expect her brother to have to deal with her hubby's family ? Those rumors of Letizia not letting Cristina's in-laws staying at the official residence is rather bizarre. Letizia's mother still lives at her own house. The national wealth even threatened a lawsuit against the tabloids for spreading those ridiculous rumors.I don´t know about this, Letizia might have thought that it would be natural for her family to stay with her future in-laws, but the same goes for Infanta Cristina as for her father, she may have to welcome her brother´s wife but she wouldn´t have to put up with her family. There was a rumour about Letizia´s mother getting some royal house, but I haven´t heard anything more about that so it might just have been a rumour.
I don´t think that Letizia´s family are the kind of people that the the royal family would normally mix with on a social level, but I don´t believe that Queen Sofia doesn´t have friends in Spain besides her sister, I am sure she has many friends but I am also sure that she is very much aware of her position as the Queen of Spain and royal protocol.
It was in Sofia's biography that she had no friends (non-family members) in Spain. She didn't have friends, only had acquaintances in Spain. One good thing is that nobody can invent stories on her told by her friends or friends who knows her friends LOL.