I've encountered conficting info on the mother of this Cristina. Most sites have her mother's name as Jimena / Xemena but seem to mix up which woman by that name was Cristina's mother.
Here on Geni her maternal grandparents are of Alfonso V 'el Noble' de León, Rey de León and Urraca de Navarra, reina consorte de León, making Cristina the maternal granddaughter of Alfonso V. However, Medlands indicates that's only an unproven hypothesis. So I Googled Alfonso V to read about his known descendants:
This is a translation from the Spanish Wikipedia:
https://es.wikipedia.org/wiki/Alfonso_V_de_Le%C3%B3n
"He married for the first time in the year 1013 with Elvira Menéndez , 19 20 daughter of his tutor Count Menendo and his wife Muniadona, 21 granddaughter on the paternal side of Count Gonzalo Menéndez "dux magnus of Portugal" and the countess Ilduara Pelaez. Two children were born from this marriage: 21
Bermudo III of León (1017-1037), King of León from the year 1028 to 1037. 19
Sancha de León (1016-1067), wife of King Ferdinand I of León —last Count of Castile—, son of Sancho Garcés III of Pamplona . 22
Elvira died on December 2, 1022. 23 19 20 The following year Alfonso contracted a second marriage, between May and before November 13, 1023, with Urraca Garcés , daughter of King García Sánchez II of Pamplona , and sister of King Sancho el Mayor , 24 despite the ties of consanguinity since both were descendants of Count Fernán González . 23 19 20 From this second marriage was born:
Jimena Alfonso . According to some authors, she married Count Fernando Gundemáriz , son of Gundemaro Pinióliz based on a donation that he made with his mother that was confirmed by Xemena Adefonsi regis filia and that this marriage, that of Fernando and the infanta Jimena, were the parents of a daughter, possibly named Cristina, who was the mother of Jimena Díaz , the wife of Rodrigo Díaz de Vivar . 25 However, it is stated in a Portuguese diploma dated in 1045 that the wife of Fernando Gundemáriz was Muniadona Ordóñez, daughter of Ordoño Ramírez —great-grandson of Count Gonzalo Menéndez— and his wife Elvira. 26 2728"
So the key info there is in the last paragraph from the above excerpt where it indicates Fernando Gundemariz wasn't married to the daughter Jimeno ALfonso but to Munaidona Ordonez, which is what I've been seeing on the majority of sites, including Medlands:
https://fmg.ac/Projects/MedLands/SPANISH%20NOBILITY%20EARLY%20MEDIE...
" FAMILY of DIEGO FERNÁNDEZ
1. DIEGO Fernández (-[before 24 Jul 1046]). There are two possibilities for the parentage of Diego Fernández. Either he could have been Diego Fernández, son of Fernando Gundemáriz & his wife Muniadomna Ordóñez or, secondly, he could have been Diego Fernández, son of Fernando Flaínez & his wife Elvira Peláez. Discussion about the relative merits of each case appears to revolve around the description of his daughter Jimena as "nieta del rey don Alfonsso [indicating Alfonso VI King of Castile], filla del conte don Diago de Asturias" in the "Corónicas" Navarras (as well as in other narrative sources)[491]. Menéndez Pidal, in order to explain this family relationship, hypothesised that Diego´s [second] wife “Cristina” was the daughter of Fernando Gundemáriz and his supposed wife “Jimena, daughter of Alfonso V King of León”[492]. This approach would be consistent with the second possible parentage noted above, but as discussed in Part F of the present chapter under Fernando Gundemáriz, there appear to be several difficulties with the hypothesis. A more straightforward solution is the first possible parentage noted above, which would satisfactorily explain the family relationship with King Alfonso VI and does not require any speculation about the parentage of Diego´s second wife. Conde. "Diaco Fernandiz comes" confirmed the charter dated 19 Jan [1026] under which "Stephanus presbiter" donated "ipsa corte" to the monastery of San Andrés, San Facundo and San Martín in Tolia, Zamora[493]. “...Didacus Fredenandiz comes...” subscribed the charter dated 30 Dec 1028 under which Vermudo III King of León donated “villam...Cordario” to Santiago de Compostela[494]. The 24 Jul 1046 charter of his daughter Oneca [Mayor] appears to indicate that her parents were both deceased at that date. m [firstly] ELVIRA Ovéquiz, daughter of conde OVECO Sánchez & his wife Elo ---. Her parentage and marriage are confirmed by a charter dated 24 Jul 1046 which refers to property which “comitissa domna Gelvira et…viro suo Didaco Fredenandi” inherited from “comite domno Oveco et domna Eilo”, another charter dated 19 Apr 1058 specifying that “conde Oveco Sanchez” was father of Elvira[495]. [m secondly [CRISTINA], daughter of [FERNANDO Gundemáriz & his wife Muniadomna Ordóñez]. Torres refers to a copy of a document at Oviedo, the original of which has disappeared, which names “Cristina” as the mother of Jimena, daughter of Diego Fernández Conde [in Asturias] and wife of Rodrigo Díaz "el Cid"[496]. Menéndez Pidal hypothesises that Diego´s wife was the daughter of Fernando Gundemáriz[497]. As noted above, another possibility is to identify Diego Fernández with the son of Fernando Gundemáriz (see above). It should be noted that it is not necessary that Conde Diego married a second wife at all in order to explain the various family relationships which are specified in the charters quoted in this section.] "
This passage explores the complications of trying to prove who Diego Fernandez married and the important info is at the bottom of the above excerpt:
" [m secondly [CRISTINA], daughter of [FERNANDO Gundemáriz & his wife Muniadomna Ordóñez]. Torres refers to a copy of a document at Oviedo, the original of which has disappeared, which names “Cristina” as the mother of Jimena, daughter of Diego Fernández Conde [in Asturias] and wife of Rodrigo Díaz "el Cid"[496]. Menéndez Pidal hypothesises that Diego´s wife was the daughter of Fernando Gundemáriz[497]. As noted above, another possibility is to identify Diego Fernández with the son of Fernando Gundemáriz (see above). It should be noted that it is not necessary that Conde Diego married a second wife at all in order to explain the various family relationships which are specified in the charters quoted in this section.] "
So, if I'm understanding what all this is saying, it's 'more likely' that Cristina's mother wasn't a daughter of Alfonso V but was Muniadonna Ordonez but even that is just the base-case hypothesis and neither mother is proven beyond doubt. So I'm wondering if there is other more definite info on who her mother was that would resolve the controversy.
Thanks in advance to anyone who may have additional documentation that I haven't come across.