Is their interference causing you or your wife to doubt your decision? Is it causing resentment or anger in your wife towards either you or them?
Fortunately, when I choose to become Catholic (I was formerly methodist, so I was not subjected to the same hardcore anti-catholicism as your in laws) my family supported my decision as one that I made for myself as an adult. I did have some anti-catholic friends who tried to dissuade me, though. It can be hard to have someone trying to undermine your choice especially when they are doing it in an underhanded fashion.
On to some things that might help you…Pray for them, pray for your wife, and pray for God’s guidance. Talk to your wife to see if she feels anger, or resentment towards you or her parents. Find out if she is having doubts, pray together about the situation, and for God’s guidance. It may become necessary that if this progresses or causes trouble in your marriage that you will have to cut contact with them. It may also be that her parents are controlling and believe that she must still obey them.
Has your RCIA group covered the sacrament of Marriage yet? If not, hopefully they will soon or go together to talk to your priest regarding the Church’s belief about this beautiful sacrament. If you have discussed it, talk to your wife about it, what it means to the two of you, possibly even talk to your priest with what you know, and what it means to you and about the situation with the in laws…I’ve received a great deal of good advice from my priests in confession and in general discussions. (my inlaws and I don’t get along well 9 times out of 10, and a visit with them usually leads me to confession, where I gain a lot of insight and a better way of dealing with our conflicts).
If the in laws begin to cause conflict, anger, or resentment amongst you and your wife, it may be time to cut off any and all contact with them until one or two things happens (or preferably both) 1. they recognize that you and your wife are adults, and that you are married, and as such you and your wife make decisions together with the help and grace of God. 2. they accept that they no longer are in control of the daughter’s life, that she is an adult, and that her decisions do not need their approval.
My own in laws have had some difficulty seeing my husband as an adult, and that as a married adult, he does not need nor need to seek their approval on our decisions. Things are improving though as time goes on, and as they see that we are not going to consult them on our decisions. My husband very much believes that he left his mother and father when we got married and formed a new life and family with me. Although our parents and siblings are still family to us, they are different from the family we have become. This took some adjusting for both of our families, but it is coming along…
sorry for writing such a long bit…
I wish you a happy journey into our Faith, May God’s light shine on you both!