It is possible that enlarging a bathroom and adding a spa tub will make it necessary to re-enforce the supporting walls. But it is impossible to say whether this is necessary without a structural inspection.
The way you enlarge the bathroom and the size of the enlargement will determine whether you can support the addition on posts (like a deck), treat it as a bump-out, or you will need to build a complete foundation. When you add a spa tub as part of a remodel, the factor you must consider is the weight of the tub itself, which will be determined by the size and the material of the tub, the size and weight of the motor(s) and pump(s), and the weight of the water the tub will hold.
The safest way to make the assessment of need for reenforcing the supporting walls is best made by a structural engineer after a careful inspection. You will find that it is well worth the minimal cost to have peace of mind and a safe structure.