Nicea, I have come across that, with friends, and it is difficult. It's not clear how well you get on normally.
Certainly begin by explaining how you feel and ask if they have any ideas (I expect you've done this already). I have 3 suggestions to explore, all of which I have known to be done, and which may or may not suit:
1. The house is rented out - agree that you either use an agency, or one of you takes responsibility and is paid for that. You all share the cost of getting it up to standard for renting. It won't bring you a lump of cash, but may be an option you can live with.
2. Your siblings buy you out in one of 2 ways:
By handing over a sum of money, and adjusting your share of the house accordingly.
Say a house worth £3,000, so £100k each. A gives you £50k & B gives you £10k.
A now owns 50% of the house
B owns approx. 36%
You still have £40k invested in the house, approx. 14%
I think that adds up, but you get the picture - and finally when sold, it gets distributed accordingly.
Of course, there are legal costs to this, which may make them think!
OR the siblings get a mortgage to buy you out completely - which really make them examine their 'investment'.
Hope you sort something out. Unless the property is in an unusual area, it is unlikely to gain significantly - they can always use their share to buy property if that's what they want to do.