Welcome to Rootschat Samantha

As Jim has stated, your information is a little confusing. If I have read it correctly we have:-
Great Grandma (bio) giving birth to your grandmother and her brother in 1944 in Hull
Great Grandma then gives birth to a second set of twins in 1945 in Gainsborough.
Husband of Great Grandma looks after your grandmother's brother. Your grandmother is living where and what happened to your grandmother's brother?
Your grandmother's birth certificate, and presumably her brother's, does not name the father so one has to assume that great grandmother's husband was not your grandmother's father.
Great Grandma therefore had an affair twice? whilst her husband was away.
The distance between Hull and Gainsborough is either 43 miles or 52 miles depending on what route you take. Seems to be quite some distance to go to have another set of twins. One wonders why she didn't stay in the Hull area.
As Jim has said, what is it about the Gainsborough births that makes you sure that Great Grandmother is the mother of these twins?
You have not provided us with your Great Grandmother's name, so we can't do any checking for you to see if we think the same thing.