I’d suggest getting to know both women. Take them both out to eat and have a long talk with both of them. After all the other sister will be your new sister in law so she will be a part of your life whether you like it or not. Hopefully they will both become comfortable with you and will open up and you will better understand their different and unique personalities.
On twins. It depends. Some were raised practically as one unit hardly spending a day away from each other. Others have developed their own personalities and can be quite different from the other. Either way you need to get to know them.
A girl who is a twin is no more or less likely to be uncomfortable during sex as any other girl, unless she had physical complications during birth. Your grandparents and neighbors are wrong.
A twin is not more likely to have twins (or at least not significantly).
As with any marriage, communication about sexual matters is key. Talk to her.
It’s likely, but by not means certain, that your wife will have a particularly strong relationship with her twin sister. It likely won’t get in the way of your relationship with her, but if it does, communicate your concerns with her.
The only way to know her thoughts on the matter is to ask her. There are no generalities that apply to all twins, and there are no significant differences between twins and non-twins when it comes to sexuality. So strangers on the internet can’t help you here.
Not aiming to fan the flames here, but this thread got me thinking a bit (which can be a dangerous thing…)
I have never been in a relationship with an identical twin, but I could imagine some of my feelings of closeness and attraction for my wife getting unintentionally transferred to her twin. After all, they look alike, are genetically identical, and may have quite similar personalities. So it is not impossible that I might find myself with more feelings for the twin than I might otherwise have for a non-twin sister-in-law.
And if I were a jealous type, imagining some other guy sleeping with the twin might be a bit unsettling. Might.