Are Hollyleaf Jayfeather and Lionblaze really siblings?

Sort of but it is confusing. But this could change because Brambleclaw and Squirrelflight are not mates anymore.

Leafpool is Jayfeather, Hollyleaf, and Lionblazes's mother. Squirrelflight is Leafpool's sister. Brambleclaw is (well was) Squirrelflight's mate. Brambleclaw's father is Tigerstar. Mothwing's father is Tigerstar.

So they are sort of related but barely.