My mom thinks my half-sister isn't her child

I found out recently that my mother wants to have a maternity test done because she thinks there was a mix up at the hospital and my half sister isn't hers.

Understand my half-sister is in her 30s with 4 kids. So seeing that after 3 decades she wants a dna test I was rather surprised to find this out.

She cited that she's wondered for years, she said she never had that mother's bond with her like she had with me, she said she looks nothing like her, and they are completely different people with no similarities. This is true my mother is a hard working individual while my half sister's picture can be found in the dictionary next to the definition of fuck-up

She said she knows there were several cases of baby switches around the time she was born so she's wondered for years. And that she informed my half sister 2 years ago that she wants a dna test. She just has't done it yet.

I was rather surprised to hear all this.
is it normal that my mother after 3 coming close to 4 decades wants a dna test?