In response to changes in relative humidity, wood expands and contracts. There is no stopping this reaction. Applied finishing materilas, at best, slow the change in size and shape, but do not prevent it.
In general, wood does not expand or contract equally in all directions: it is an "anisotropic" material. The unequal response is what results in splitting and deformation of shape - twisting and cupping. For the purposes of analysis, three directions or orientations are defined: linear, along the length of the fibers of the wood; radial, from the center of the tree outwards, and; tangential, around the circumference of the tree. Wood always expands and contracts least along the length of its fibers/"grain", so little as to be negligible in most applications. The concern is its radial and tangential expansion and contraction.
These two websites gives an overview of the terms: http://workshopcompanion.com/KnowHow/De ... vement.htm
http://www.popularwoodworking.com/trick ... -expansion
Average values are tabulated for the % change in size, in both radial and tangential directions, for many species of wood. These indicate that for a specific width of board, cut with a specific grain orientation (e.g. flatsawn vs. quartersawn) you can expect the tabulated % change in size for each change in wood moisture content of 1 %. The smaller the per cent change, the less the wood will expand or contract. The ratio of radial to tangential change is also tabulated and provides a measure of how likely the wood's shape will distort in response to changes in relative humidity.
With that background, Dalbergia sissoo, commonly called Northern Indian rosewood, Sissoo or Shesham, has R, T and T/R values of 3.1%, 5.1% and 1.8, from http://www.wood-database.com/sissoo/.
Dalbergia latifolia, commonly called East Indian rosewood, has R, T and T/R values of 2.7%, 5.9% and 2.2, from http://www.wood-database.com/east-indian-rosewood/. East Indian rosewood is one of the most common materials from which guitar backs and sides are made - as well as fingerboards and bridges. It is considered a relatively stable wood.
Comparing the values of sissoo and latifolia, they aren't hugely different and have similar stability to changes in moisture content. (Compare that, for example, with Lignum Vitae, at 5%, 8% and 1.8, or Gabon ebony, used for centuries in instrument making, at 8.3%, 11.2% and 1.3. It is common to see cracked ebony fingerboards, for example.)
The numbers above give a general indication of what to expect, useful for determining how a finished wooden article will behave to changes in humidity. However, if the wood is properly dried before being made into finished objects and the grain orientation of those objects chosen to minimize wood movement, the effects of changes in humidity on the finished objects can be minimal.
Guitars have a variety of specific issues. One issue is that the large, thin wooden surfaces of the top and back are fixed in size all around their edges: they are constrained, preventing expansion and contraction of the overall size of the top or back. The result is that when a guitar top or back is subject to high humidity, the top or back bows to increase its size, since it cannot increase its perimeter. When a guitar top or back is subject to low humidity, it shrinks. The top or back can only shrink to a flat surface, providing the shortest distance across its width. When the forces causing shrinkage exceed the strength of the wood fibers, the top or back splits to relieve those forces. An unconstrained wooden sword, for example, will not have that issue. Movement across the width of a guitar top or back is minimized by using quartersawn (vertical grain) wood. It has now become popular - with increasing scarcity of "good" wood - to use flat sawn wood for backs and sides. Doing so increases their risk of cracking, if the humidity of the wood is not maintained.
So, in short, the sissoo will probably work quite well for wooden weapons, and guitars, if the wood is properly dried prior to making the weapons and has its grain oriented to suit the specifics of the finished wooden object. (For guitars, the ideal grain orientation is quartersawn, not possible unless the tree is at least 16+ inches in diameter, or the top or back is made of more than 2 pieces. For weapons, such as swords, the ideal grain orientation might be "rift" sawn, by definition, neither quartered nor flat sawn.)
EDIT:
Some examples of sissoo for use in guitars:
http://rctonewoods.com/RCT_Store/indian ... od-c-1_38/
http://www.vorreiterguitars.com/?page_id=804