Question: Who is Euphrosyne Doukaina Kamatera's sibling-in-law?

Knowledge 1: Euphrosyne Doukaina Kamatera
Euphrosyne Doukaina Kamaterina or better Kamatera (– 1211) was a Byzantine Empress by marriage to the Byzantine Emperor Alexios III Angelos.Euphrosyne was the daughter of Andronikos Kamateros, a high-ranking official who held the titles of "megas droungarios" and "pansebastos" and wife, an unknown Kantakouzene.She was related to the Emperor Constantine X and Irene Doukaina, empress of Alexios I Komnenos.Both of her brothers had rebelled against Andronikos I Komnenos; one was imprisoned and the other was blinded. [SEP] Knowledge 2: Alexios III Angelos
Alexios IIIAngelos  (1211) was Byzantine Emperor from March 1195 to July 17/18, 1203.A member of the extended imperial family, Alexios came to throne after deposing, blinding, and imprisoning his younger brother Isaac II Angelos.The most significant event of his reign was the attack of the Fourth Crusade on Constantinople in 1203, on behalf of Alexios IV Angelos.Alexios III took over the defense of the city, which he mismanaged, then fled the city at night with one of his three daughters.From Adrianople, and then Mosynopolis, he unsuccessfully attempted to rally his supporters, only to end up a captive of Marquis Boniface of Montferrat.He was ransomed, sent to Asia Minor where he plotted against his son-in-law Theodore Laskaris, but was eventually arrested and spent his last days confined to the Monastery of Hyakinthos in Nicaea, where he died.
Knowledge 1: Alexios III Angelos
Alexios IIIAngelos  (1211) was Byzantine Emperor from March 1195 to July 17/18, 1203.A member of the extended imperial family, Alexios came to throne after deposing, blinding, and imprisoning his younger brother Isaac II Angelos.The most significant event of his reign was the attack of the Fourth Crusade on Constantinople in 1203, on behalf of Alexios IV Angelos.Alexios III took over the defense of the city, which he mismanaged, then fled the city at night with one of his three daughters.From Adrianople, and then Mosynopolis, he unsuccessfully attempted to rally his supporters, only to end up a captive of Marquis Boniface of Montferrat.He was ransomed, sent to Asia Minor where he plotted against his son-in-law Theodore Laskaris, but was eventually arrested and spent his last days confined to the Monastery of Hyakinthos in Nicaea, where he died. [SEP] Knowledge 2: Basil Doukas Kamateros
Basil DoukasKamateros was a Byzantine aristocrat and senior official.Basil was the son of the official and theologian Andronikos Doukas Kamateros, and brother of Empress Euphrosyne Doukaina Kamatera, wife of Alexios III Angelos( r. 1195 – 1203).A relative of the imperial family—his grandmother Irene Doukaina was probably a daughter of the" protostrator" Michael Doukas, brother- in- law of Alexios I Komnenos( r. 1081– 1118) —he held the high rank of" sebastos", and by 1166 held the office of" protonotarios".By 1182 he had advanced to the post of" logothetes tou dromou", but was dismissed, blinded( apparently only in one eye), and banished to Russia when Andronikos I Komnenos( r. 1182–1185) took power.He returned to Constantinople and was again" logothetes tou dromou" under Isaac II Angelos( r. 1185 – 1195), and remained active at court under his brother- in- law Alexios III.After the Fourth Crusade he fled to the Empire of Nicaea, established by his nephew Theodore I Laskaris, who in 1209/ 10 sent him on an embassy to the King of Armenian Cilicia, Leo I.
Knowledge 1: Euphrosyne Doukaina Kamatera
Euphrosyne Doukaina Kamaterina or better Kamatera (– 1211) was a Byzantine Empress by marriage to the Byzantine Emperor Alexios III Angelos.Euphrosyne was the daughter of Andronikos Kamateros, a high-ranking official who held the titles of "megas droungarios" and "pansebastos" and wife, an unknown Kantakouzene.She was related to the Emperor Constantine X and Irene Doukaina, empress of Alexios I Komnenos.Both of her brothers had rebelled against Andronikos I Komnenos; one was imprisoned and the other was blinded. [SEP] Knowledge 2: Robert Vadra
Robert Vadra is an Indian businessman, entrepreneur and the husband of Priyanka Gandhi.He is the son -in- law of Sonia Gandhi and Rajiv Gandhi, and the brother- in- law of Rahul Gandhi.

Answer: Isaac II Angelos