Formula in linear mixed model (fitlme)

I wanna analyze difference of brain size between controls and patient.

But it is very hard to me because there are some siblings in data.

I would like to see the group difference accounting for sibling effects.

    Group Age Br_size Subj_ID Family_ID
    1     14    100      1      1
    1     17    113      2      2
    1     19    124      3      3
    1     16    102      4      1
    1     12    119      5      4
    1     19    124      6      5
    .     .     .        .      .      
    .     .     .        .      .      
    2     19     99      45     28
    2     13    118      46     2
    2     13    120      47     29
    2     19    112      45     30
    2     13     98      46     31
    2     13    100      47     3

For 47 subjects, 15 subjects have no relationship to others but the others have least one sibling in this data.

How do I make a formula? I think..

    fitlme(tbl, 'Br_size ~ 1+ Age + Group + (1 + Subj_ID  |Family_ID );

Is it right? I am looking forward to your response and comments.