One of two executors of fathers will, did not sign, but was witnessed (England)

I was named as an executor of my father's will along with my sister in law (brothers wife), she was also named as having control of fathers finances /medical etc In The event he became incapacitated (I don't know the technical term). Recently my sister was reviewing the will as dads health isn't great, and she realised that although my section was signed and witnessed, my sil's section wasn't signed but strangely was witnessed. What does this mean for the will? Does it invalidate the entire thing, or does it mean my sil can't be co-executor?

Thanks in advance