Ruling on shaking hands with one’s mother’s maternal aunt

Your mother’s maternal
aunt is also a maternal aunt for you. And you are a mahram for her, so there
is nothing wrong with your shaking hands with her. 

Allaah says, explaining who is a mahram (interpretation of
the meaning): 

“Forbidden to you (for marriage) are: your mothers, your
daughters, your sisters, your father’s sisters, your mother’s sisters…”

[al-Nisa’ 4:23]

The word khaalah (mother’s sister or maternal aunt) in
the verse includes the maternal aunt of one’s father and mother, and
grandfathers and grandmothers, and the same applies to paternal aunts. 

Al-Sa’di said (p. 244): 

‘Ammah (paternal aunt) refers to the sister of your
father and grandfather, no matter how far the line of ascent reaches, and
khaalah (maternal aunt) refers to the sister of your mother and
grandmother, no matter how far back the line of ascent reaches. End quote.
See also Tafseer al-Qaasimi, 5/86 

It says in Zaad al-Mustaqni’, when discussing which
woman are mahrams: Every paternal aunt and maternal aunt, no matter how far
back they go. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in
his commentary: “No matter how far back they go” means the maternal and
paternal aunts of one’s father, or the maternal or paternal aunts of one’s
mother, or the maternal aunt of one’s grandmother, or the paternal aunt of
one’s grandfather or grandmother, and so on ad infinitum. End quote from
al-Sharh al-Mumti’, 5/184.