Complex sentence: Unfortunately, it was also largely what pitted her and Vladimir against her other children (including her daughter-in-law Anna, Tchaikovsky's niece), who claimed, among other things, that he was pocketing from the company for his own use.
Split into simpler sentences: Unfortunately, it was also largely what pitted her and Vladimir against his siblings and sister-in-law, Anna (Tchaikovsky's niece). They claimed, among other things, that he was pocketing from the company for his own use.