If a person does not offer Ud-hiyah although he can afford to do so, he is missing out on a lot of reward that results from offering the sacrifice, but he is not sinning, and he does not have to make up for what he missed in years past.

Based on that: There is nothing wrong with you sharing the purchase of the cow with your relatives, so that you will have one seventh of the cow, which you will intend as an udhiyah – but less than one seventh will not be acceptable as a sacrifice on your part – and they will dispose of the rest of it in whatever manner they like, whether for a wedding feast or otherwise. But we should point out that the minimum age of a cow for it to be acceptable as a sacrifice is two years; anything younger than that will not be acceptable, even if it has a lot of meat. Please see question no. 41899. And Allah knows best.

Scholars have different views on the ruling of udhiyah. Most of the scholars are of the view that udhiyah is Sunnah, and is not obligatory.