Weak inner thighs are a major problem for large swaths of the population. And the main problem with having weak inner thighs is that if you suffer from this unfortunate condition you will bear weight through the outside of the body rather through the middle, or core.
When we are standing up straight it would be nice if our feet were planted evenly on the floor. An evenly planted foot means that the weight is spread equally across the ball of the foot and the heel. The very middle of the foot as well as the inner arch should not be making contact with the floor but the rest of the foot should.
Most people I meet and work with have feet that tend to bear weight towards the outside. If this is the position that you find yourself in the odds are you have an imbalance between the inner and outer thighs. And it is likely that other muscle groups will suffer as well.
Many people with weak inner thighs also have tight dysfunctional peroneal muscles. If you bear weight towards the outside of the foot you can assume that the outside of the shin where the peroneal muscles attach, are also pulled to the outside.
I often teach basic heel lifts in my yoga classes and invariably students have trouble keeping the inner foot grounded as the heel lifts. This is an indication that the peroneal muscles are pulling your shins out to the side. If that is the case, your inner thighs which are meant to keep the legs together (adduct) when the heels come up, will be pulled in the direction of the outer shins and outer foot.
Learn to use your peroneal muscles as intended and it will help in your quest to develop your weak inner thighs into strong happy inner thighs. It might take a while to make these changes but in the long run your legs, as well as your upper body and spine will benefit greatly.