My son is getting married. His finance is having her 2 sisters in the wedding and 3 best friends. My son has only 1 sibling and that is his sister. My husband and their father passed away 2 years ago and there is only the 3 of us. Do you feel that my daughter should be asked to be one of the bridesmaids? I really appreciate your answer as soon as possible. Thank you for your help.

Yes I do.

Usually sisters and brothers on both the groom and bride sides are asked first to be bridesmaids and groomsmen.

So the bride should ask your daughter to be a bridesmaid, just as your son should ask her brother(s) (if she has any) to be a groomsmen.

No I don't.

While I think it would be a nice gesture for your daughter to be a bridesmaid, the bride should have who she feels closest too.

Personally (I was married 6 months ago), we decided to have a very small wedding with only 1 attendant each. I have 2 sisters, my husband has one living brother. His brother was his best man, yet my matron of honor was my cousin. I had 2 very good reasons for my decision... 1. my cousin is very close to me. 2. My sisters are younger (22, 17) and I didn't want to burden them with the stress/cost/etc. In the end they actually thanked me for not having to be "stressed out" about it.

It's a personal choice and while I'm sure some of my guests thought it looked odd not to have my sisters... it didn't matter to me or my siblings... I did what worked for all of us.

Why not suggest that your son ask his sister to stand up on his side? Its becoming quite popular to have "mixed sides" standing at the altar. Since she would be standing up for him, I think it could be quite endearing. :)
You might find this article helpful Alternative Attendant Formations

No I don't.

I think it is up to the bride who the bride's maids are. Granted it would be a very polite gesture for the bride to ask your daughter to be a bridesmaid, it isn't required. Perhaps your daughter could be a decorator or a server at the reception or even the guest book attendant. (the person that makes sure everyone signs in)

When I got maried I wanted my brother who I am very close to to be my maid of honer, but he refused reminding me that he is a guy, so he just stood up their with us, if your son and duaghter are close, talk to him about it she could stand on his side not a traditional thing I know but who cares ask your son about it, the wife may ask her to be a bridesmaid if she realizes her husband wants his sister, incadently my cousin was my maid of honor,and only bridesmaid even though I have 2 sisters

It would be a nice touch but this is the bride's decision. Don't set your daughter up to have any expectations that she will be in the wedding party. If you see it is hurting her feelings, perhaps you can privately talk to your son. Maybe he can tell his fiancé, "it would mean a lot to me for my sister to be an official witness to our marriage, so how would you feel about having her as one of the bridesmaids".

If your daughter and son aren't close, or she and the bride don't get along, don't push it. They will have to work out their relationships over time on their own.

As for an expectation she should be in it, I just wouldn't set yourself up to feel that way.