This is an excellent question and you’re smart to choose your next steps very carefully. I’ll give you my thoughts, with the caveat that if you ask someone else, they might have different but equally valuable advice.
First thing’s first, I would suggest that you and your boyfriend need to have an honest conversation. His recent decision to call off your plans to get engaged, and his recent absence while helping his sister, may be a signal that he’s questioning your future together and is pumping the breaks with his actions rather than telling you outright. However, you can’t know for certain where this decision comes from unless you talk to your boyfriend.
Moving in with someone is a big decision and something that should never be entered into lightly. If you’re frustrated with the recent dynamics in your relationship, imagine how much more difficult it will be if you add financial and domestic stressors into the mix. Additionally, if you move in together and find that the relationship is reaching a breaking point or a dead end, it will be that much more emotionally difficult to end the relationship while both you and/or your partner also need to find another place to live, all while balancing possible financial constraints, such as bills and a lease.
In regards to your boyfriend’s sister, while it’s completely understandable for you to feel upset by her recent requests of his time and attention, you should keep in mind that it is actually your boyfriend who is choosing to agree to what she is asking of him. You may want to reflect on the events at Christmas. Is the fight you got into after your boyfriend’s father’s remarks now making your boyfriend question your future together? Again, you won’t know the answer to this question until you have a conversation with your boyfriend about it.
If you decide to request a conversation about the recent events with your boyfriend, I would suggest that you put aside your anger and resentment, take a few deep breaths, and set up a time to speak with him. When the agreed upon time comes, rather than focusing on problems you see in his father or sister, allow yourself to be vulnerable and focus on explaining the emotions that are underneath your anger.
For example, rather than blaming his sister for her requests, explain that when you don’t get to see or talk to him as much, it makes you miss him and feel sad because you love him so much and enjoy spending time with him. By showing your vulnerable side, your boyfriend will likely respond with empathy. You’ll also want to understand and validate your boyfriend’s feelings. Remember, even if you don’t agree with his, or his family members’, perspectives you can still do your best to understand how he feels to come to an understanding that way.
Finally, once you’ve been able to understand each other’s perspectives and feelings, ask for what you need and see if you can come to a compromise that you both can agree to. Remember to be reasonable and not demanding. Perhaps you can agree that moving forward you will have more honest conversations with each other when conflicts arise and can then reconvene in a few months to talk about your future plans together.
I hope this answer helps and I wish you and your boyfriend the best!