bring food to her house - it will probably offend her (since food and nuturing are so closely connected for her), but since you are going to be over there often, it is really your only practical option. Also plan to eat before/after going to her house, and bring snacks for visits. Talk to her about it though - and include her, by asking her to 'help you guys by keeping kosher snacks in the house for you (or you purchasing them and keeping them at her house - whatever works).
Tell her that this is something you believe in strongly, and you dont want to NOT eat at her house (which is the only other alternative), and can she come up with a solution? (very effective way to get people involved - present the problem as you see it, not as they see it, and ask them for a solution). If she doesnt come up with something reasonable, suggest that you will bring food to her house to heat up during dinner time and you will take care of your family and she can take care of hers and you all can eat together.