Katy: Let's see, according to the document (on this declaration only, more information may come clearer on the other witnesses accounts) Fernando Campa treat Antonio Mier as uncle, I think that Fernando Campa is close in age/generation to Fernando Mier and they likely treat each other as cousins.

Antonio Mier treat Juan Campa as Uncle which make me think that he (Antonio) was a generation younger.

Putting both relations together we can assume that Fernando Campa (the Count) was a grandson of Juan Campa (the Knight)

I couldn't read the last sentence regarding this accounts but it seems as if the relations between the Mier and Campa are not known to be "consanguineous", but because the witness know them to treat each other as family he assumes that they actually are.

I made a transcription of this documet

Transcripcion del documento en este archivo (Jul 8 2015, Dr. Moises Beutelspacher Huizar)

https://familysearch.org/pal:/MM9.3.1/TH-1-18381-23144-5?cc=1874591&wc=…
En la Villa de Xerez , en quince días del mes de Abril de mil setecientos setenta y cuatro años (1774), ante el Sr B Don Joseph Thomas Alon de Larrastegui, Cura Bene por su Magnificencia, pidole que de esta dicha Villa Vicario Jun(an) Cele(sti) no Onella, por el Ilustrisimo y Reverendisimo Sr maestro Don Fray Antonio, Alcalde del orden sagrado de predicadores Dignisimo Obispo de este Reino de la Nueva Galicia , el de Leon y Provincias de el Nayarit, Californias, Coahuila y Texas, de el Consejo de su Majestad (¿? ), se presento el subsecuente escrito:
Peticion
El Benemerito Don Felis Joseph Rodriguez Saenz, presbítero domiciliario de este Obispado y residente en esta Villa de Xerez: a nombre de Don Fernando de Mier Theran y Mijares mi hermano (

The little bits I can understand from that page are:

Don Antonio de Mier y Theran married Doña Margarita Mijares y Solorsano, their kid (the subject) is Don Fernando de Mier Theran y Mijares.

Dn. Antonio Mier y Theran parents life is unknown to the witness since they where from Burgos. Dn. Antonio lived also in "el Epasote" part of Colotlán.

Da. Margarita Mijares died when her kid born. Her parents were Dn. Diego Mijares y Solorsano and Da. Rosa María de Luna. (Fernando's Grandparents) from that same village (Jerez),

Dn. Fernando Mier Theran y Solorsano got married with Da. María Salvadora Rodrigues Saens (who died a year earlier to the declaration) She was the sister of the Bachiller Dn. Felis Joseph Rodrigues Saens.

The witness stays the "pureza de sangre" of Dn. Fernando's Grandparents (that they are not Judios, Morors, etc.)

Dn. Diego Mijares y Solorsano (maternal grandparent) was Uncle in second with third grade of "Yllustrisimo Señor" Flores Obispo; Here I think it could

Diego Minjares Solórzano fue tío en segundo con tercero grado de sanguinidad del Ilustrísimo Señor Flores, obispo de Nicaragua…

This bishop Flores probably is the same as José Antonio Flores y Rivera who according to the website (http://www.manfut.org/leon/obispos.html) was bishop of Nicaragua for a short time (1755-1756):

año de 1755

El Ilmo. Sr. don José Antonio Flores y Rivera, Canónigo de la Sta. Iglesia de México: tomó posesión de la mitra de Nicaragua por febrero de 1755, y murió por diciembre de 1756: sepultóse en su Catedral.