Extension of Tourist Visa beyond visa expiry

[Confuseddesi]

I'd appreciate it if anyone could clarify a few things for me. My mother in law is due to visit me this month. She has a tourist visa valid from July 18 to December 18' but she will only be using 3 months of that visa. My wife is due to give birth in december and we would have loved for my mother in law to stay to help out with taking care of the new born especially in the first month after delivery.

I know that the rule states you can only extend a tourist visa if the total duration of stay is less than 6 months. i.e if a visa was issued for 3 months then you can extend to 6 months as long as the actual stay within the country does not exceed 6 months. But I wanted to know would the fact that my mother in law is only in the country for 3 months of the valid 6 month visa help in extending the visa or is it out of the question to extend a 6 month issued tourist visa?

Ps. For those wondering, we did not delay application of the visa because my wife was quite sick at the beginning of her pregnancy and we actually needed her mother to come to the UK to take care of her, but unfortunately we were unable to get her here due to other unforseen circumstances.

Any help/suggestions would be greatly appreciated