I still feel Eshonai being dead was a mistake. I wish he could develop the sister relationship like he did in Warbreaker, with both of them being alive.
But since we can't change that fact, let's talk about pros and cons of every possible route Brandon could take:
Venly-only flasbacks
Pros
-Develop Venly as a character
-More Cosmere secrets
-Gives context to whatever Venli is doing in the present
Cons:
-Eshonai as a pov character is finished, which is sad and unsatisfying.
Eshonai-only flashbacks
Pros
-More Eshonai development, which feels needed after her abrupt death.
-Counterpoint between Eshonai in the past and Venli in the present
Cons:
-I feel if Eshonai takes the flashbacks Venli could suffer as a character. Remember how much Dalinar flashbacks were relevant for his character development.
Venli is only a secondary character at this point, without flashbacks she could be found lacking compared to the Shallan, Kaladin and Dalinar.
Both
With both you have all the pros and cons, with the risk of some of the pros being too little and some of the cons being too heavy, depending on the execution.
You take the risk of readers feeling they didn't get to know any of the sisters sufficiently. In the other hand, if executed great, you can construct the sisters' past with more depth.
Nonetheless, Brandon could make Eshonai be heavily featured in Venli flasbacks (or vice versa). If Venli things about her sister and the chapters themselves gives us a look into Eshonai's personality, then maybe will be satisfied even without flashbacks from one sister pov or the other.