Why do you use the word disproportionate?
You make it sound as if it is your mother's function to favour you over your two other siblings.
Presumably, there is a reason why you and your children are living with you mother. This could be because she needs looking after or because she provided you with accommodation when you could no longer stay in your own home. For whatever reason.
You invested money in your mother's house. And now you say that your mother is leaving everything to the three of you. So - was that always her intention? In which case, you knew about it when you moved in.
So, was there no long term conversation about your situation when you moved in? Did you assume that because you'd be living there, that she would leave the house to you?
And what difference does it make if your siblings have their own homes? Should they be penalised and you rewarded because of that?
The tax you refer to is not CGT but CAT - capital acquisitions tax - the tax paid to Revenue when you are given a gift or receive an inheritance with a value over a certain threshold.
See this site for more info:
http://www.revenue.ie/en/tax/cat/thresholds.html
So, when your mother dies, unless she changes her will in the meantime, you will receive one third of her estate. Will this be enough for you? Can you ask to take one of the investment properties as your share and you live there?
Ultimately, for reasons you may not have gone into, you could, potentially, seek to overturn your mother's will if you believe you are entitled to a greater share than she leaves you but you should educate yourself now on a person's freedom of testation (as in, children have no automatic entitlement to ANY share of their parent's estate). Your mother could decide to leave it all to St. Vincent de Paul or the ISPCA.
mf