the whole idea is stupid though and a cheap tactic for drama, because loving someone is mostly out of your hands, so the set up of it stands out more and give us that (AGAIN?) response.
I think that sort of restriction can make sense if you have the right sort of witches. Namely, you'd need something more spiteful, where the witch is fueled by hatred or revenge and actually falling in love would require both a fundamental change in what they are and abandoning their prior purpose in life.
The girls we've seen so far are pretty normal, if a bit catty, and nothing like the type of monsters I'm talking about, so it comes off as incongruous.
The only other explanation I can think of is more liked to ritual purity and virginity, though that's generally more associated with divine powers. If that were the case, then it's not so much the act of falling in love as it is the deed love leads to that costs your magic. (Un)fortunately, this explanation seems even further from the story we're reading.
Anyway, I think we can safely assume that the restriction doesn't apply if it's Super Special Chosen Soul Mate Lovers. Which the protagonists obviously are.
Actually, I don't think the restriction really exists, at least not for those girls and not quite as literally as implied.
Already, we can very clearly see that Alice has feelings for Mari, but Miriam also seems to have feelings for Alice. If simply falling in love cost you your powers, than I think they'd both already be powerless, but neither seems to be having any issues.
Instead, I think the line has more to do with the origins of the current witch culture, more than the girls themselves. The story paints a picture of a group that was unfairly hunted and persecuted, who used spite to save themselves, pouring thing their hatred and rage into a grudge they could nurse while bending knee to their former hunters. It wouldn't surprise me if more than a few of them were the type of witch that I was talking about before, but that's not the reality the current generation of witches lives.
It's possible the grandmother was talking about those older witches without realize the world has moved on and the new generations no longer need to draw strength from that grudge.
It's also possible that she wasn't talking about magic. While she mentions it in the beginning, she only says a witch will lose their "powers" at the end. This society seems to place a great deal of importance on working up towards some future revenge and they mostly seem to be amassing political power, while working closely with their intended victims, to accomplish it. It's probably a lot harder to commit the kind of betrayal they're talking about, or intentionally tip the world into chaos, if you love others, especially if pulling the trigger would endanger them. A witch who builds positive emotional bonds would endanger that power, more than anything.