I think you should go ahead with the shidduch for the 18 yr old. It is unfair to withhold a shidduch from her because she has an older sister. I am in the same predicament (have a younger sister in shidduchim and I am in shidduchim) and would be absolutely horrified if I was the reason that prevented my sister from even meeting a shidduch possibiity.
You, or someone else, needs to speak to the older daughter, because this seems to be a deeper issue. Your 19 yr old needs to come to terms with the fact that life events do not always play out the way we’d like. And this is probably not the last time she will face something like this (although we all wish it was).
By the way, involvement lessens feelings of jealousy. Meaning, if the older daughter is quite involved in the goings-on of the shidduch, it will help her overcome her hard feelings. (Contrary to the instinctive reaction of hiding things…)