Sororities can easily be (and usually are in some degree) a ticking time bomb- you have to make sure everything is perfect before attempting, and even then it's still a gamble.
Seeing your tank, you really do need a lot of plants, it's pretty sparse, giving them no shelter, nor broken lines of sights for the female being chased to easily escape. I would advise to separate if possible, or buy a lot more plants (I'm talking about making it hard to see them when glancing in the tank). In the future, make sure to have everything you need before you decide to buy more bettas- as they can be addicting, but one must be able to properly home them before buying them- they are living animals that have requirements.
You know I like ya, and I know you've had troubles over the last few months.. I'm not trying to be rude or mean, just making sure you understand the consequences, which sadly, you have today. :( I'm sorry you lost your girls.
You will need a clean/new rubber band to attach the sponge to the outside of the intake portion, or even use a new panty hose to cover it up.
Are you sure she died because of the filter? As it's rare for the suction to be that strong to keep hold of a fish- normally they die elsewhere, and when sinking to the bottom get sucked up to the filter.
Make sure to do a large water change, at least 50% (if it's been a week or less after doing last water change), if it's been longer then a week since your last water change, do a 25% as soon as you can, and another 25% the next day to avoid osmotic shock.