My sister-in-law and her family are planning to spend Pesach with us. Her husband learns day and night. They have next to nothing and are considered aniyim. Having them for the entire Chag will be quite a financial burden for us. Are we allowed to use maaser money towards feeding them.
Maaser money can certainly be used for this purpose. The most basic cause in the allocation of maaser money is the support of those who study Torah without sufficient means, and putting up your needy family for Pesach certainly qualifies.
For more details on allocation of maaser money, please see the piskei halachos on the topic, which can be found (fully in Hebrew, and partially in English) on the site.