There's no Morgause in Fate, instead Gareth is also a daughter of Morgan, so yes, Mordred and Gareth are sisters.
Aye. Gawain, Agravain, and Gaheris are their brothers. I think a good amount of images on this site are missing familial relation tags, mainly these characters. Also for example, if Gawain, Mordred, and Artoria were all in one picture, wouldn't it have the following tags: Siblings, Brother & Sister, Aunt & Nephew? Since Mordred is Artoria's only child, I think that makes her the aunt of Morgan's other children?
Aye. Gawain, Agravain, and Gaheris are their brothers. I think a good amount of images on this site are missing familial relation tags, mainly these characters. Also for example, if Gawain, Mordred, and Artoria were all in one picture, wouldn't it have the following tags: Siblings, Brother & Sister, Aunt & Nephew? Since Mordred is Artoria's only child, I think that makes her the aunt of Morgan's other children?