Future sisters in law who are not bridesmaids

That's thoughtful of you to consider a gift, but I would just gift them once at their own wedding. I'm sure they'll appreciate cash! Btw, sisters-in-laws do not expect to be bridesmaids and that relationship does not have to be reciprocal. Honestly, it takes much money and energy to be a bridesmaid, so they may even decline as they, themselves, are also in saving and planning mode.

Sure it can be nice to welcome having new SILs but, as just said, it is okay they are not bridesmaids. You select the bridesmaids from those you honor (and desire to have next to you) of people that have been part of your life (which mostly is before marriage). Beyond that, it might fit to have a SIL or someone from the partner at your side, but this is not demanded by any etiquette.

I'd get them a corsage to wear at the wedding. Thats very sweet of you to think of them. Some other options would be a bracelet or earrings, a nice candle, something you found helpful for wedding planning, or a bottle of wine to take some of the stress off wedding planning. Good luck!

A gift is not necessary, nor is it necessary to make them bridesmaids or other roles if there is no relationship. Just give them a corsage. After the wedding, go out for lunch if you want to get to know them at that point.

Agree with the above, a corsage (and maybe a nice note!) is perfect. You could also take a photo of just you and future SILs and then recreate the photo at each wedding!