Who is that special person that your waifu loves the most, besides you?
And is there canon info to support your answer, or is it your own interpretation?
In Flandre’s case, the answer seems obvious to me. It has to be her onee-sama big sister Remilia. Although there is no real canon info to support this interpretation, it seems to be the most plausible answer. As there is little reason to believe that Flandre’s parents are there for her (I guess they have been dead for quite a long time…), Remilia is therefore her only close family left. And I guess that after about 500 years living together, they must have gotten really close.