It seems to me to make perfect sense for Medina to conjure Huddibras and
Sansloy by their love for their mothers in this way (that is, not by their
love for their ladies' mothers). She goes on to conjure them by their love
for their ladies, and their love for their queen (we assume, on the
Spenserian model of, say, Amoretti), which constitutes the standard appeal
to the three types of social bond (natural, erotic, social/political) that
we see so often rehearsed in Book IV. I don't think Medina is necessarily
trying to conjure them by shared allegiances, except insofar as they both
have mothers, ladies, and queens.

If you would nurse at fullest breasts of fame... (Astrophel and Stella).


>         Whilst thus they mingled were in furious armes,
>                 The faire Medina with her tresses torne,
>                 And naked brest, in pitty of their harmes,
>                 Emongst them ran, and falling them beforne,
>                 Besought them by the womb, which them had born,
>                 And by the loues, which were to them most deare,
>                 And by the knighthood, which they sure had sworn,
>                 Their deadly cruell discord to forbeare,
>         And to her iust conditions of faire peace to heare. (FQ 2.2.27)