If there are no strings attached to the gift, accept it gracefully and use it for whatever. Invest, buy stuff for the house, whatever you want to use it for.
And then if the inlaws ever need your help, give them your money or time as needed, because you love them and appreciate them and their kindness over the years.
In healthy families, this is a wonderful dynamic. In unhealthy ones, money/gifts are used as manipulations. If this is a healthy family, then cherish the gift that was given because they are so happy and proud of you, and know that being there for them in the future is a good thing for both of you and your relationship.