Sorry for late reply. Yeah, most of Bengalis are something like "Gujrati Mid-caste (like Patels) + A bit of Indo Aryan + East Asian according to geneticist Razib Khan. I know the story of Hazrat Shah Jalal and his 360 followers. Maybe some Sylhetis are their descendents, Since they took local wives and their descendents were assimilated into Bengali community, their descendants now will only get traces of Yemeni ancestry likely around 2-3% of whole genome only. And if someone from Sylhet get Y haplogroup like E1b or J1, he is likely a descendent of Hazrat Shah Jalal's companions. I've seen somewhere, some Sylhetis from UK doing a DNA test and posting their results, and all of them looked similar to average Bengalis results, also none of them had any Arab Y haplogroups like E1b or J1.
Fair skin or Afghan look that your mom got is actually very local. Sounds surprising, but Afghans are genetically closer to south Asian according to geneticists. We Bengalis are carrying those traits from at least 7000 years. Also, most of my family members are pretty west Asian looking and are lighter skinned.
Read these articles:
https://www.gnxp.com/WordPress/2018/03/31/the-maturation-of-the-south-asian-genetic-landscape/
https://www.biorxiv.org/content/early/2018/03/31/292581.article-info
P.S. As for your family book containing Farsi scripture, Persian was the official Language of Bengali sultanate, and Mughal Bengal, so it's not surprising at all. How old is your family book?