I have two problems with this
1 why did you make both Shimmy shake and lighthoof both have dads when according to your next gens magical lesbian spawn is possible
2 if you assume that shimmy shake and lighthoof both have dads and are step sisters why do you also assume both holiday and lofty divorced their husbands well that is possible their is the possibility that at least one of scootaloo’s aunts might’ve been widowed or not married until when scootaloo’s aunts got married.

1. Precisely. The parents of the “Next Gen_eration” are the ones who can benefit from magical science progress (Shimmy & Lighthoof are young mares, likely around the ages of the Mane Six in season 1), not the case yet for Lofty & Holyday).  
   [MLP:FiM_S3E05] -> When unicorn!Twilight (aparently) managed to turn a mare into a stallion, alicorn_amulet!Trixie said that was something “impossible” [given her capacity to cast some “highest level unicorn” spells, it implies that kind of spells were not developed/researched yet].
    
2. It could also be the case that they are the result of magical experimentation on re-shufling of DNA/cloning (Lofty & Holyday working as “test subjects” in their youth…)  
   -> There is no point on taking into consideration every posibility (Occam´s Razor for more reasonable/likely implied backstory relationships).

In fact, Scootaloo, Shimmy Shake and Lighthoof are cousins.