Visa for In-laws

[gc_psraj]

I am in the US on H1B, my wife is on H4. We are planning to invite my mother-in-law who is nearly 60 to visit us for a while. We also want my mother-in-law to be accompanied by my wifes sister, who has her family back home, and she works as a teacher. My question, when I send an invitation letter, should I send the invitation or my wife ? And the affidavit 1_134, should it be signed by me or my wife? Since my wife is on H4, I am not sure how to handle the situation. We also have a son who is born in the US. Would appreciate any inputs.

Thanks.
gc_psraj