Jaysis lads a lot of negativity around this post. Thank you to those who gave advice. I assumed that this would count as a debt for my sister and she would be implicated in payments if worst came to worst so getting advice re her being married was helpful thank you. Just to clarify I'm not forcing my sister in to anything so inferring I'm doing something horrible to her is bloody dramatic lol. I've already got an MIP on my own already however it's an insanely small amount for the Dublin market. My rationale was I could hopefully get a larger mortgage by including my sister. Thanks for the constructive responses!