How long shold I Wait before applying for another family member visitor visa

[MimiQ]

I applied for my Mother Visa in May,2013 ( she got the 10 year visa after getting rejected twice before!!). Now I want to apply for my Father's B2 visa as well so he can visit me. My Mother is visiting me right now and is leaving by end of August. Does it going to create any issue if I apply for my Father visa now while my Mom is here visiting me? My Father passport was not ready by the time my Mom applied for the visa. My father does not have any fixed trip date yet though and planning to come when my Mom visit me next time.

My sister also wants to visit me. It is better to apply my Father and sister visa together or they should apply individually? And if they apply individually, how long I should wait once I applied for my Father visa?
Please advice.
Thanks
MimiQ

[peace999]

When your mother applied for the visa, what did she say to the officer why she is going alone without her husband?

You have to give more details about your sis; single/ married, student , employed for how long, salary?