She vowed to give her jewellery in charity – can she give it to her sisters?

The basic principle concerning one who vows to give something
in charity without specifying a recipient is that he should give it to the
poor and needy, and those who take from zakaah because of their need,
because they are the people who are entitled to charity; poor relatives are
more entitled to it than poor strangers. See al-Mughni. 8/209 

Shaykh al-Islam Ibn Taymiyah said in al-Fataawa al-Kubra
(5/554): 

If he vows to give some wealth in charity, he should give it
to those who are entitled to zakaah. End quote.  

Based on that, if the sisters asked about are poor and needy,
it is permissible to give the jewellery to them, unless the questioner
intended to give the charity to specific poor people. 

The Standing Committee for Issuing Fatwas was asked: I made a
vow and I fulfilled it, but I gave to my brothers and sisters, who are poor.
Have I fulfilled my vow?  Please note that I vowed that if Allaah blessed me
with such and such, I would give a month’s salary. 

They replied: 

If you mentioned the poor in your vow without specifying
anyone in particular, then your poor brothers and sisters are more entitled
to it than anyone else, so there is nothing wrong with what you have done.
But if you specified a certain type or intended such in your vow, then it is
not permissible to give what you vowed to anyone else, and you have to give
the equivalent of what you gave to your siblings to the poor to whom you
intended to give. End quote. 

Fataawa al-Lajnah al-Daa’imah,
23/391 

And Allaah knows best.