Is it permissible not to fulfill a vow that stipulated wearing a jubah and abandoning skirts and trousers, knowing that 15 years have passed since the vow was made and that it is difficult to wear a jubah in the summer?

A woman wearing tight clothes (like tight skirts and pants) is impermissible, and repentance from it is immediately obligatory.

As for your vow to wear a wide garment upon your brother's recovery, it is an obligatory vow that is not binding. What is obligatory upon you is to wear wide clothes and abandon tight ones. You are not required to offer expiation for breaking this vow.

If you are already wearing wide clothes and vowed to wear a specific garment, this is also a permissible vow that is not binding, and breaking it does not require expiation.