Scott/Sarah's mother?

I feel like something like that is likely, as well, but not for the sibling's motivations as much as Alec's. He was among the first to travel with Jon Grissom through the Charon relay (Relay 314) and be attacked by turians, so I could certainly see Alec being:

1. A bit xenophobic if his partner was killed by aliens as well as personally being attacked without provocation.

2. Motivated to leave the Milky Way/less attached to staying.

3. Incredibly irritated, to say the least, that basically the exact same thing happened to him again - a peaceful endeavour being mercilessly attacked upon unknowingly entering the territory of an alien species. I could even see that leading to him being in an antagonistic role with more extreme ideas about how to establish a homeworld in Andromeda (genocide-type ideas). Or maybe you can agree with him. Or maybe he'll just be very cynical.

I'm all for more options, especially about motivations, but it wouldn't really make sense for the sibling to have not wanted to leave. They're an adult and wouldn't be obliged to, and wouldn't be forced. So I agree with shechinah, the best you can hope for is an option to regret the trip, which I'm sure we'll get.

The only reason I don't think that would work is that we'll likely see Jieng's (I think that was the spelling) face at some point, leading to complications of ethnicity. I'm not sure why it would matter that she has a different last name though. Plenty of women keep their own surnames even now, and I definitely wouldn't see that as odd in 2185. Also, as you said, they might not be married or even together anymore.

Her story is fine, but not how the siblings were influenced by her, because that's a breach of roleplaying.

It wasn't just Shepard in Bioware games. Hawke, too, and some of the other backgrounds in DA have both parents alive even if you don't meet them.


Anyway, there is another possibility: Alec is a single dad and never wanted a partner. Through either surrogacy or advanced science, there was never really a mother in the picture.

You could be forced along. For example if you are in the military your father outranks you and he can order you to go. 
But if we are stuck with those few options you say, great 'my' character is already not my character and instead a predetermined protagonist.

If the mission is only for those who volunteered, then I don't see why dad would order his kids to go.

Seems so. Bioware did it with the Inquisitor, but I guess that was too much to expect out of the Mass Effect team. 
If what the others are saying is true, we won't even get to do what we did with ME2 since we'll be stuck with only being totally for this mission. 

Well Bioware suggested that some people involved knew about the imminent Reaper invasion. Your father is one of the highest members of the Initiative so would most likely be one of those who do know, so could force his kids along as a way to protect them from the Reapers.

And I would like that to be an option instead of Ryder being stuck as "We're going to Andromeda! This is cool! Thanks dad!". 

pfff...... MCs don't have mothers,stupid!

How is it different from Shepard being ordered to go down to Eden Prime, or the PC of Dragon Age Origins being forced to drink darkspawn blood with no option to fight Duncan?

There are limits to how open a VG RPG can be made. This is not a TTRPG that we are all sitting down to play. I'm not saying that I can't see your point; I just think your desires often seem to be just out of reach.

Interesting thoughts posted here!
I was wondering if Jian is actually their Aunt and she had their mother killed to insure Alec and the kids came along?

She's just as much in love with Alec but her sister... huh I could see her wanting them somewhere safe after all if that was their mother then she KNOWS about the Reaper invasion and indoctrination so it would make more sense she faked her own death and persuaded her sister to go along to insure her family survived?!

If their mother was Cerberus then maybe so is Jian and she had her own reasons for wanting Alec and the kids only her sister's link to Cerberus was discerned and whilst she was able to prove her loyalty, Alec and the rest on Hyperion couldn't so they assumed they were Cerberus and arranged an accident to explain why the entire Ark went off course?

Some scary stuff indeed!

The mission launches after ME2 and before ME3.