League of Legends (LoL) Question: is this samira ,sivir sister?

As far as we are aware, no. Sivir doesn't have much childhood references in her story and it has yet to be updated. Samira's story gives insight on how she became what she is today. So unless we get a lore update on Sivir, I still doubt Samira and Sivir would be blood related. Although, they have a very similar professions but have different goals in mind, Sivir being in it for the gold while Samira being a mercenary for fun.

Nope. They just happen to be both from Shurima and both mercenaries.

According to the lore, Sivir and Samira does not seem to be related. It wouldn't make sense regardless if they were.