Fellby and Sinsby can do a poly relationship? Yes, no? Maybe so?
Remarkably, yes. Most of the Grillbys would actually do fine together. They are at their core, similar enough to get along, while differing enough to not stagnate. What’s more, they’re all, as a rule, exceedingly old. While they would not likely pair up amongst themselves, if there were a partner that made more than one of them happy, and was interested in pursing each, they would not be resistant. They know each other remarkably well, and are not going to give up a shot at happiness over “not wanting to share”. While they do tend to be possessive in their own ways, there is an in-group cohesion that allows for them to share amongst themselves. Do not expect, however, for your options outside the fire elementals to be very good.
Sins in particular is open minded enough, while also being quite comfortable and confident with himself, that polys are fairly easy for him. Fell is one of the more possessive, but largely from a place of terrible loss. However Sins is able enough to handle Fell’s moods, and Fell trusts Sins abilities to keep what is ‘theirs’ safe. In this particular dynamic, Fell would not feel the need to assert himself over Sins as he might with some of the others, so long as there was no attempt at domination over him. And Sins simply isn’t the type to “over run” any one within a relationship.
So ultimately, yes, these two can fairly comfortably poly, and so can most of the other Flame Babes.