In the unlikely possibility you haven't seen Endless Waltz... be warned Spoilers abound...

Since Mariemaia's too young to consider the possiblities of a romantic pairing, we'll skip that part and just do a section on her family. 

Mariemaia's Mother:  Leia Barton.  Leia was a pretty woman with a shoulder length, gold colored hair, apparently pleased to have had a daughter.  Unfortunately she died, means unknown, only two years after Mariemaia's birth so it's unlikely she remembers her mother at all aside from any picture she may have.

It's possible Leia might have known about the plans for making her daughter "The Ruler" of earth. I thought this because of something her brother says:

"Her name is Mariemaia, she's my sister's daughter. She's going to be the leader of earth after we conquer it," --Trowa Barton, Leia's brother.

It's interesting to point out Mariemaia's unique looks.  She has red hair, whereas the majority of people in her family are blondes.  Leia - Blonde, Treize - Blonde/Brown, Trowa - Blonde, Dekim wasn't a blonde, but neither does he have red hair. 

Mariemaia's Father:   Treize Kushrenada.  Although Mariemaia's father is listed as "unknown" in her file when Heero looked her up, she says many times she is "Treize's daughter".  Mariemaia seems to have a higher regard for her father, than for her mother... although she'd been mislead by her grandfather as to what Treize's true ideals were, it seemed her ultimate goal was to make him proud of her... to gain his approval.   I think it  likely he'd be proud of her for growing up so strong, even though she did threaten the peace he himself had had to die for to attain...

It's unknown whether or not Treize was ever aware he had a daughter.  It is confirmed however Mariemaia was conceived during the period when Treize was injured in battle in space combat, where he met Leia Barton at a hospital way before he was the Oz Specials Commander.  

Extended Family:

Mariemaia's Uncle: Trowa Barton

It's unlikely she had any kind of close relationship with him seeing as he died right before the beginning of Operation Meteor.   Trowa Barton was much like Mariemaia herself, intelligent and over confidant, not to mention they were both short tempered.

Mariemaia's Grandfather: Dekim Barton

She was closest with her grandfather, but not emotionally close.  I say this because in the movie she doesn't call him "Grandfather" (and even Dorothy called Duke Dermail grandfather...).   Mariemaia called him "Dekim" and he called her "Ms. Mariemaia..." so I'd say there was no emotional loss for her when he died.  And even less so after he shot her and then said he could easily create a replacement... also saying how he 'created' her... 

These things must have been pretty upsetting for a young child and so it was to her advantage that Lady Une had infiltrated the Brussels Presidential Residence to save her life... It was Lady Une, a complete stranger, who showed that she cared for the girl, whereas everyone else at this time was rebelling against her...