Her knees no longer can support her weight.
My grandma is overweight and her knees can´t support her additional weight.
My grandma is overweight and her knees can´t carry the heavy load put on them.
These are ways to express that, but sentences #2 and #3 can be seen as impolite to many, so it would be used in limited contexts (perhaps explaining to a doctor).
Sentence #1 shows more empathy, I think.
As for the word choice: I don´t think one can say the knees support/can´t support overwheight, they would support weight on them.
Or something overweight on them, like an overweight body, for example.