My husband is Armenian and I am Mexican. My husband speaks a good amount of Spanish and we both speak English. I have 2 amazing stepchildren, 9yr old girl and 11yr old boy. They only speak English. We have twins on the way and are trying to figure out how to teach them all three languages. We would love to teach them all three languages from the start but are trying to figure out the best way. We were thinking of my husband speaking Armenian to them, I would speak Spanish to them and our other two children speaking to them in English. The only tricky part would be when we are all together what language do we speak considering our other two children only speak English. In addition to that I am an elementary teacher and would love to teach them their letters, letter sounds and numbers in English before going to school but will that confuse them since I speak to them in Spanish?
You can speak whatever language you'd like between you, your husband and your stepchildren, including when your twins arrive. But it would be best for each of you to speak your twins in your first languages. The science shows that growing up in a bilingual or multilingual environment may cause some small delay in your children starting to speak, but that they will be better at language and at learning languages once they start speaking. This is a skill that they will keep for the rest of their lives and will also help with other new languages that they learn. You have plenty of time to help your very lucky children to become proficient in speaking English, before worrying too much about how to help them read and write it! What an exciting adventure for your new children, and for you. Enjoy!