Only your sister-in-law knows the real reason she did this, of course - although even she may not understand her deeper motives. Our minds and emotions are complicated, and sometimes we may not even realize why we act the way we do.
One reason for your sister-in-law's actions might be guilt, because she knows she could have done more to be a good wife. Often both the husband and the wife are at fault when a marriage turns contentious (although not always, of course). Perhaps as she looks back she wishes she'd done more to bring harmony to her marriage, and her lavishness is a way of trying to make up for her failures.
She also may be expressing genuine grief over the loss of her husband. Although their relationship wasn't perfect, she misses him, and there is now an emptiness in her life she may never have expected. She also may be grieving because she knows she will never again have an opportunity to say, "I'm sorry" to her husband, or to try to heal the problems they had.
Pray for your sister-in-law; she's going through a harder time than you may realize. Pray especially that she will turn to Christ, and find the forgiveness and peace He alone can give her. Jesus said, "Peace I leave with you; my peace I give you. ... Do not let your hearts be troubled and do not be afraid" (John 14:27).