In Ahmadiyya history books, you won’t see much of mention of MGA’s sister, not the twin-sister that MGA was born with, this was another older sister.  There are 2 quotes from the infamous Seeratul-Mahdi here.

The story
pdf page no. 118 / 296
Narration no. 127

This is a long narration that talks about start of Muhammadi Begum episode. I will not translate all due to its length, but will translate some important points:

” Humble one states that Masih Moud’s sister was married to Mirza Muhammed Baig. But Mirza M. Baig died early and our aunt (mgaq’s sister) had to spend rest of her life as a widow. Our aunt used to get revelations/dreams. Masih Moud’s cousins i.e. Mirza Nizam ud Din and Mirza Imam ud Din’s sister Umar an Nisa was married to Mirza Muhammed Baig’s younger brother Mirza Ahmed Baig who gave birth to Muhammadi Begum. Mirza Nizam ud Din and Mirza Imam ud Din were worst kind of faithless and athiest in nature. Mirza Ahmed Baig was under their influence and was following their footsteps. Those people were always demanding a heavenly sign from mgaq because they were against / disliked Islamic teachings. 

There used to be a cousin of mgaq who was lost for a long time and his property was transferred to his wife Imam Bibi. She was sister of Mirza Ahmed Baig. Mirza Ahmed Baig wanted to transfer that property to his son Muhammed baig, brother of Muhammadi bagum. But Imam Bibi could not transfer that propery by law unless mgaq’s consent. Therefore, Mirza Ahmed Baig very humbly and respectfully requested mgaq to sign on the tranfer deed. ”
pdf page no. 225 / 296
Narration no. 199

” Narrated to me Mirza Sultan Ahmed through Molvi Rahim Baukh M A that father (mgaq) had a sister who used to get dreams and visions very often, but grandfather had the opinion that she had mental problem. The she saw some dreams that made grandfather change his opinion. Once she saw an old man with white beard in her dreams who gave her a piece of paper as peryapt ( Taweez) with something written on that. When she woke up she had piece of leaf with few verses of Quran written on that.

Then she saw another dream that she was walking in a river upon that she got frightened and screamed water and woke up. Her calf were wet and there were spots of fresh sand. Grandfather said, this has nothing to do with menat fault.”