Lord Shiva is the ultimate Brahman of the Universe. Mata Sati after her immolation took rebirth as Mata Parvati in the family of King of the Mountains Himavat and Queen Menawati.
Adi Shakti took incarnation as Mata Parvati in order to win Lord Shiva as her husband.
She is the younger daughter of King Himavat and Queen Menawati. However, Devi Ganga is their elder daughter. This, makes Ganga her elder sister.
As per Valmiki Ramayan , Maa Ganga is elder sister of Maa Parvati/Uma.
तस्यां गंगेयमभवज्ज्येष्ठा हिमवतः सुता |
उमा नाम द्वितीयाभूत्कन्या तस्यैव राघव ||
This Ganga has emerged as an elder daughter to Himavat through Mena, oh, Raghava, and that way a girl renowned by the name Uma has become a second daughter to him.
~ Valmiki Ramayan 1.35.16
Words of Sage Vishwamitra :—
Oh, best one among men, River Ganga is the eldest daughter of Himavat, and oh, dextrous one, you have to offer water-oblation to the departed paternal-uncles of yours in her waters, namely the holy waters of River Ganga’.
~ Valmiki Ramayana 1.41.19
Words of Lord Brahma :—
This Ganga is the one with snow-broth, the eldest daughter of Himavat, and oh, king Bhageeratha, god Shiva alone is capable to sustain her force in the course of her alighting onto earth, and in fact, he is to be commissioned for that purpose.
~ Valmiki Ramayan 1.42.23
Ganga Devi becomes the Sister in Law to Lord Shiva.
However, King Himavat being the lord of the mountains any river originating from it becomes his daughter and sister in law of Lord Shiva.
In the Harivamsa, for example, Mata Parvati has two younger sisters called Ekaparna & Ekapatala. They are also Sisters in law to Lord Shiva.
Thanks.