It's great that you are going to help your daughter over the summer! Parent involvement is an important part of a child’s success.
Research shows that learning to read in a first language is easier than learning to read in a second language. Once you learn how to read in your first language, you can transfer those skills to a different language. In her case, if she speaks Spanish at home with you, it may be helpful to focus on building those reading skills in Spanish this summer. When she goes into first grade, she can focus on transferring those skills to English, rather than learning them for the first time.
If she is more comfortable in English, work on those activities in English. Once she gets stronger, you can start practicing letters and words in Spanish. You can also try both languages and see which one is easier for her — after all, you know her best!
Finally, don’t hesitate to expose her to both languages over the summer through different activities and bilingual books, music, games, and movies.