Q:

Who inherits beside the daughters if the mother has siblings?

If a mother dies leaving only two daughters. The two thirds of her wealth will certainly pass to her daughters.

Allah instructs you concerning your children: for the male, what is equal to the share of two females. But if there are [only] daughters, two or more, for them is two thirds of one's estate. (4:11)

But will the other third pass only to her siblings who are (still) alive (in the present case only two sisters are alive) or to the heirs of her death siblings too?
And how this third will be distributed? 

A:

After distribution of the fixed shares, all of the remainder goes to the عصبة .

ألحقوا الفرائض بأهلها فما بقي فهو لأولى رجل ذكر
Give the shares to those who are entitled to them, and what remains over goes to the nearest male heir.
— Bukhari and Muslim 

In this case the closest eligible blood relations would be brothers, and sisters in the presence of either brothers or daughters. Siblings are closer than the heirs to any dead siblings (nephews) so they will receive the entire remainder and deprive the nephews.
If the siblings include only brothers, the estate will be divided equally between them. If they include brothers and sisters, then a brother will receive twice the share of a sister:

وإن كانوا إخوة رجالا ونساء فللذكر مثل حظ الأنثيين
If there are both brothers and sisters, the male will have the share of two females.
— Quran 4:176

If they are only sisters they will become عصبة in the presence of daughters, and the estate will be equally divided between them.

قضى فينا معاذ بن جبل على عهد رسول الله صلى الله عليه وسلم: النصف للابنة، والنصف للأخت
Mu`adh bin Jabal gave this verdict for us in the lifetime of Allah's Messenger (ﷺ). One-half of the inheritance is to be given to the daughter and the other half to the sister.
— Bukhari , باب ميراث الأخوات مع البنات عصبة
أقضي فيها بقضاء رسول الله صلى الله عليه وسلم للبنت النصف، ولبنت الابن السدس، وما بقي فللأخت
Abdullah said, "The judgment I will give in this matter will be like the judgment of the Prophet, i.e. one-half is for the daughter and one-sixth for the son's daughter and the rest of the inheritance for the sister."
— Bukhari , باب ميراث الأخوات مع البنات عصبة

