Since the items in question are a gift to you, they are your possession, you are certainly free to throw them in the trash. Personally, I would agree that discarding the items is appropriate.
Be aware, your mother-in-law may very well have sentimental ties to those items. Therefore destroying those items will likely provoke a hostile reaction. But, they are not her possession, therefore she has neither ownership nor claim to their care.
Realize this, the question at hand is not whether some other person (ie. family member) will be pleased with what you do. Rather it is how you will personally deal with receiving and possessing idols.
If you received an anonymous free gift of a Hindu Shiva statue, would you keep it? I should think not. So why the confusion?
If the circumstances were otherwise, for example, someone were offering you an idol statue then you could simply refuse and not take possession of it. However, the circumstances of this gift is not in question of your acceptance — it is already yours by legal right.
Whatever you decide to do, for you are certainly at liberty to transfer the ownership to your mother-in-law, it will be wise to discuss that such gifts are unacceptable. However, I should imagine that (if you should choose to do so) explaining why you destroyed those items like the O.T. judges and kings of Israel did in their time would make for a particularly enlightening conversation and settle this issue regarding your conviction of faith once and for all.
It would also be wise to recognize that, if you do throw those items away, you are not throwing them away because of the faith of your father or mother-in-law. Do not allow this to be taken as 'I will destroy this because you are Roman Catholic'. This must be an entirely personal conviction of your own heart before God.
Also notice, another vital factor, it is almost certain that those devotions are not recognized by your Roman Catholic family members as idols or objects of vain superstition. Depending upon the specific beliefs your family holds and your ability to reason and evidence from Scripture concerning these devotions will have the greatest impact; at least, insofar as your effort is concerned.
For I know from experience that the common Roman Catholic does not belief Mary is their Savior. Rather they believe praying to the saints is no different than asking our fellow church-members to pray for us. This is something for which you will need to prepare yourself to give a sound answer.