Perhaps this is more of a hashkafic question.
If someone's Father-In-Law comes to them asking that they should sign to be an arev on a very big loan (that the Son-In-Law has no way of paying back) is he required to sign on the basis that it's his Father-In-Law and should do this favor. Or can he decline since l'maseh it would be impossible to ever pay back. Is there a difference if there are others that signed that do have the ability to pay back or even if the others that signed also don't have the ability to pay back.
Answer:
The Gemara (Yevamos 109b) writes that one should distance oneself from arvus, and the Rambam (De'os 5:13) applies this across the board. However, there are some loans that a person can't receive without guarantors, and it is certainly a mitzvah to help people receive loans by guaranteeing them.
This applies the more so for family, of which the pasuk writes "mibesarcha al tis'alem."
Signing arvus implies accepting responsibility for a certain share of the loan. A person can sign even without the wherewithal to pay, but he should know that he will be responsible for raising his share (proportional to the number of areivim).
If he thinks he won't not be able to pay his share, or even raise the money, he should generally not sign. If the loan is secure, and the borrower is one's father-in-law, there is still room to do his bidding. For family we go out of our way...