Mother's Sorrow is a set that provides a static buff so you want to have it on all the time. Since you need to put Maelstrom Destro Staff on back, that means that you should have all 5 Mother's Sorrow set pieces on body or on jewelery.
Siroria on the other hand is a set that generates stacks that last for a while. This means that you don't have to have all 5 set pieces active at all times. That is why it is preferable to use it for front bar weapon.

As to which set you should have on body and which on jewelery, we have to take into consideration whether you have gold jewelery and gold body pieces. If both of them are purple then there is no difference and you can mix match however you want.
However since upgrading to gold is much cheaper for body pieces than for jewelery, many people run with everything golden except of jewelery. In that case I can't provide you with a definite answer on which set you should have on body and which on jewelery. To decide that I would have to make two versions of your build in a build editor and compare stats or maybe even make both builds on PTR and test rotations ten times for both build variations.