Going to end up in a marriage union with Castille

Playing as Portugal and both my sons died. My daughter just happens to be queen consort of Castille and is in line to inherit the crown when I die.

I have a grandson who could be in line if I change the inheritance laws.

So is this a bad thing or a good thing? It feels like I will be a junior partner in a union with a stronger country and I should avoid it, but there is nothing in game warning me against it.