Selling at a loss and buying a new property advice

I was wondering if anyone could advise me in relation to selling my current property which is in negative equity and buying a new property?

I had bought an apartment joint with my sister and we currently have an interest only mortgage on this at £127,000. The current value of the apartment is approximately £70-80,000 maximum meaning we are in negative equity.

An opportunity has just come up for purchasing my late grandparents house at £135,000 which I would be keen to do. I'm just wondering if there is any possible way I can manage to do this?

I would need to sell my apartment but obviously there is the £50-60k shortfall which I would need to make up. Is there any way I could try and sell and then move my mortgage to the new property and increase it if need be to make it up to the £135k?

I'm not sure how easy or wise it would be to hold on to my apartment and try and rent it out and get a second mortgage for the house?

Any general advice or suggestions around this would be much appreciated!

Thanks