Complex sentence: Maj-Lis was actually the sister of Kalervo Palsa's childhood sweetheart Maaret for whom he maintained a long-lasting unrequited torch of love, quite openly writing about it in his letters to her and her sister and eventually sisters husband.
Split into simpler sentences: Maj-Lis was actually the sister of Kalervo Palsa's childhood sweetheart Maaret. For whom he maintained a long-lasting unrequited torch of love, quite openly writing about it in his letters to her and her sister and eventually sisters husband.