“A wife always must switch to her husbands minhugim.”
There are times when this isn’t true. I honestly don’t know where to find the halachic sources for this though. Its generally true.
In general, the people I know who married geirim took on their wife’s minhagim because they had no real established ones.
Now as to the OP: if you are talking about eating at their house? What my aunt does is that they cook kitniyot in specific pots and she doesn’t eat those dishes. Kitniyot is NOT like chametz. In fact, infant formula is kitniyot and we were told to just wash bottles in different sinks from eating.